Win32 (Windows 95/98/2000/NT/XP) executable (EXE/DLL/etc...) compressor.
The compressed executables decompress themselves at run time, and can be used just like the original uncompressed versions.
Petite also adds virus detection to the compressed executables - they will check themselves for infection every time they are executed.
Some versions use anti-debugging and import table and PE tamper checking tricks.