The regular full scan will scan all drives the OS is aware of that are not removable or networked (as I understand it) as well as all files the OS can access.
The quick scans do the higher-threat areas, OS folders, etc. - if you have a drive F for example, that's only data and no OS files, I don't believe a quick scan will touch it.
Custom scans will scan only the drives YOU select. (or files you select and not those you exclude)