Editing and proof-reading are the last two steps to polish your work before you hand it in. Do not be tempted to skip them, because you are short of time or feel that they are not important. They will determine the first impression a reader (your lecturer in most cases) has of your text and you do not want them to think that it is sloppy or that you have not invested sufficient time!!
For an explanation of the difference between editing and proof-reading, have a look at the website of the Writing Centre at the University of North Carolina.
The Open University also offers good advice and a handy checklist.