Whether or not colors and/or objects bleed off the edges of your pages is a design decision, not a rule. If nothing bleeds, you don't need to include any "bleed area." If colors/object do bleed off the page, they should extend a set distance outside of the page boundary to allow for potential trimming inconsistencies. You want that area included in the output you send to print. That's the bleed area.
Slug is simply an area outside of the page boundary where you can include information for the printer such as fold marks or notes. It's optional, of course.