I can never keep straight the differences in parameters between avast's on-demand scanners and its resident, on-access providers. So I'll restrict this comment to on-demand scans.
The Help files are quite helpful on this point. If I recall correctly, the "strength" you select for such a scan is primarily directed to file types and their extensions, particularly where the two don't agree. And my apologies if I've misquoted anything, I'm working from (admittedly questionable) memory, and hopefully I'm close enough to be at least somewhat helpful.
A simple scan works strictly from file-name extensions, regardless of what's actually in the file. So if an exe or a dll has, for example, a .txt extension, the simple scan will most often skip over it simply because of the extension.
A standard scan, which is probably what most of us use most often, works the other way around, by determining for itself what type a file really is (from its content) rather than from what it's "called" (i.e., its extension).
And a advanced (extended?) scan will scan all files on your system, ignoring both file-type and extension, other than whatever specific exclusions you've set up.
