Both editing and proofreading involve a highly specialized set of skills and a keen eye for detail. They also have the same goal: to improve a piece of writing. However, in general, they focus on different elements of language. Also, editing implies a deeper revision than proofreading, which focuses more on objective errors and mistakes. Editing deals with:
Flow and overall quality
Ambiguity and clarification
Style and formatting
Are the words chosen accurate to express your ideas? Is the tone used appropriate for the target audience? Is it too formal, or too informal? Are there too many long sentences? Has the writer used too much passive voice?
These, among others, are some of the elements an editor looks at when revising a text.