Ask people this question and you will get a variety of answers, though I would suggest most would mention 'spelling', 'punctuation', 'typos' and – possibly – 'grammar'. While it is true editors do look at all these, there is more to being an editor than simply spotting a typo.
Put simply, an editor improves a written communication, making sure it is clear, correct, concise and consistent (the so-called 4Cs).
Different editing tasks often require specific skills (see 'Why the different types of editor?'), so depending on the type of publication an editor will:
Obviously the text itself will determine what an editor needs to do – for example, a children's picture book requires a different degree of editing to, say, a textbook on thermodynamics. Regardless of the text though the editor's job is the same: to produce a polished, professional, readable text without errors or inconsistencies.
It is worth noting one thing that an editor does not do, and that is rewrite the text – they may reword a sentence, re-order a bullet list or suggest a different structure, but it is always the author's text, not the editors.
Simply put, because not all texts are the same! The broadest division is between fiction and non-fiction, but within each are different genres, and texts that vary by length, complexity, structure and intended reading level (among many other things). For example, the editing skills required for a children's story book are quite different to those required for a physics textbook. While the basics are the same (language and structure) the textbook will require an editor with specialist technical knowledge.
The terms used to describe editors can be confusing at first (especially since some are used interchangeably) and there are no clear divisions between individual roles as many roles overlap and more than one role can be filled by one person. The main types of editor and their responsibilities are given below (these are terms relating specifically to the preparation of texts for publication, so don't include Acquisitions or Commissioning Editors who seek out new authors or authors to write on a particular topic).
The short answer is 'No', but you will most likely be doing yourself a disservice if you don't employ an editor. At the very least you should get as many people as possible to review your text. You could save money by getting your partner/parent/friend to proof your work, but you need to remember that while they may find typos, they are unlikely to give the text a critical and in-depth review and polish (because they don't want to hurt your feelings). As for relying on a spell checker, this article gives some pretty good reasons why it's not a good idea.
A professional editor is concerned solely with your manuscript and not only has an eye for detail, but also the experience and knowledge to spot things that might otherwise be missed.
Each editor has their own approach and the type of text will vary the approach used, but a professional editor will go through a text multiple times. First, they will quickly read the text – this not only tells them what it is about, but also gives an idea of the style employed. This reading will often show up any inconsistencies in format and layout and portions of the text that are ambiguous, incomplete or missing.
Next follows a word-by-word review, looking for errors in: spelling, punctuation and grammar; consistency, accuracy and clarity; style, formatting, missing/redundant/repeated text, missing words and clichés; and how appropriate the text is to the intended readership. Depending on the text the editor may do a factual check (for example, dates, terminology, units, names, symbols, abbreviations), a reference check (correct formatting), a check of the index/bibliography, and a check of the preliminary pages (title page, publication/copyright information, preface, table of contents, acknowledgements and lists of tables, figures and illustrations).
The penultimate step, which is sometimes combined with the step above, is to check the layout of the document (such things as headings, subheadings, pagination, table and figure legends, cross-references, text and paragraph spacing, and any visual elements that are used).
The final step is to read the document again, ideally after a break of a day or two, this time as a 'normal' reader (that is, slower than the first read, but quicker than the word-by-word read). This is to look for anything that might have been missed in the previous steps (ideally nothing should be found, but even the best editors can miss things).
One final thing: make sure the editor knows what format your text will be in (for example, hard copy, a Word document, a pdf) so you know how you will be able to incorporate their changes and suggestions in your final draft. How these changes are noted will depend your text's format (for example, a Word document might use track changes, whereas a hard copy will be 'marked up' using a set of standardised symbols).
Each editor will set their own fees, but a reasonable range would be $20-$30 an hour for proofreading, and $40-$60 an hour for copy-editing. Rates will vary depending on the type of text (an academic thesis, for example, will take longer than a parish history) and whether it is all text or broken up with illustrations.
The time spent on each job will also vary according to the text, but a rule of thumb is 40 pages an hour for proofreading and 20 pages an hour for copy-editing. The best option is to get a quote based on a sample (or the complete text) of your manuscript.
Not all editors are the same (so don't just pick one at random) and don't base your choice solely on price – look for someone with experience in the type of text you have written. If you have written a novel, look for an editor with experience in fiction (ideally one who has published their own work of fiction). If you have a non-fiction book, look for an editor with experience in what your book is about (for example, someone with a science/technology background, or someone with a humanities background).
A Style Manual (sometimes called a Style Guide or House Style) is a set of standards applied to a written document to ensure consistency in terms of style, format and design (particularly in academic publishing).
A Style Manual is not a grammar guide as such, rather a set of preferences set by a particular publisher to create uniformity across their product. The word 'publisher' here is used in its broadest sense and includes book publishers, newspapers, magazines, academic institutions, companies, government departments, private organisations, societies – pretty well anyone who produces documents designed to be shared in some way. The manual used might be generic or topic-specific, international in scope, or used only within a particular organisation (a 'house' style). Wikipedia has an article describing the variety of Style Manuals commonly in use here and this site has an entertaining overview of the major manuals you may come across.
Should you use a Style Manual? If you are writing for a particular publisher it is worth adopting their style requirements from the outset as this will save both time and expense at the copy-editing stage. If you are writing with the intention to self-publish, your options are broader. You can use a standard manual applicable to your genre, modify an existing manual, or you can come up with your own. As in most things, there are pros and cons to each option and a professional editor will be able to provide advice on the best option to suit your particular circumstances.
Even if you don't use a manual, you need to ensure that at the very least your document follows standard grammatical and spelling rules and that these are applied consistently.
Editors are often experienced in the publication process, and may be able to help with getting your manuscript ready for printing and publication. This might include where to get specialist assistance (such as designers or graphic artists), finding a publisher or publication avenue, finding a printer and obtaining quotes, and preparing your manuscript for a printer. Not all editors will offer these additional services, and you may not need any of them for your project. However, if you need some extra advice, it is always worth asking an editor!
So the writer who breeds more words than he needs, is making a chore for the reader who reads.
Self editing is the path to the dark side. Self editing leads to self delusion, self delusion leads to missed mistakes, missed mistakes lead to bad reviews. Bad reviews are the tools of the dark side.
Only the blank page needs no editing.
Writing is like riding a bike. Once you gain momentum, the hills are easier. Editing, however, requires a motor and some horsepower.
Editing is like pruning the rose bush you thought was so perfect and beautiful until it overgrew the garden.
Editing is like walking across a room strewn with rose petals and thorns. When you can walk across mostly unbloodied, you're finished.
Editing might be a bloody trade, but knives aren't the exclusive property of butchers. Surgeons use them too.
You write to communicate to the hearts and minds of others what's burning inside you, and we edit to let the fire show through the smoke.
No author dislikes to be edited as much as he dislikes not to be published.
Be a good editor. The Universe needs more good editors, God knows.