"Quick Scan. Only the possibly dangerous files are scanned, according to their extension. It means that the files with extensions EXE, SCR, COM, DOC, etc. are scanned. Within the file, avast! looks only for those viruses that infect the corresponding type of file. It means that macroviruses are not searched for in EXE files etc.
Standard Scan. Only the possibly dangerous files are scanned, according to their content. The file extension is ignored. Again, only the viruses corresponding to the particular file type are searched for."
This was copied from the help file (right click the "a" ball, Start avast! Antivirus, press F1). There is more information there, as well as in the FAQ section of the avast! website and "Users FAQ" posted by Tech in the General Topics section of the avast! forums--all very useful.
