There are a few different types of editors that provide different services. Here is a breakdown of what each type of editor does.
General Editors: These people will look at the writing overall, and will likely provide a surface-level edit. They may also delve lightly into both copyediting and developmental editing, but on a lighter scale than a specific Copyeditor or Developmental Editor will.
Developmental Editors: If you go to a developmental editor, you will find more focus on the plot or the content of the writing. This is a great option if you are looking for feedback on how a story is progressing or if you are trying to figure out how to get across an idea or a story in a way that makes sense. These people are great at helping close plot holes or pointing out missing information.
Copyeditors: Here are the people who are self proclaimed "grammar gurus," the ones who get hot and bothered when you strike up a conversation about the Oxford comma or common homonym mistakes. They also look out for inconsistencies in texts and focus on improving the readability of a piece.
Proofreaders: Little last minute mistakes that are easy to overlook if you are too close to the text can be very distracting to the reader after it goes to print. This is where proofreaders come in. They focus on the little things that will not change the layout of the piece. The proofreaders are the ones you call in when you think the text is ready to go to print to ensure that you don't have any little mistakes that may distract the reader from what you are trying to say.