For most modules there is no big difference between Normal and High,but there is a big one for Standard Shield. On Normal level it scans only opened files (the one you click on),on High it scans files that are scanned when created/modified and when they are opened. I recommend High setting for optimal protection.
For On-Acees scans (manual):
Quick Scan (scans only infectable files,virus targeting is probably on -> exe,pif,com,scr and so on)
Standard (Scans all files,virus targeting is probably off)
Thorough (All files,scans entire file not just header,virus targeting off)