7z.exe is the command line version of 7-Zip. 7z.exe uses 7z.dll from the 7-Zip package. 7z.dll is used by the 7-Zip File Manager also.
7za.exe (a = alone) is a standalone version of 7-Zip. 7za.exe supports only 7z, xz, lzma, cab, zip, gzip, bzip2 and tar formats. 7za.exe doesn't use external modules.