7z is an archive format with a very high compression ratio, see [1 ]. It is designed as an open and modular architecture, supporting different compression methods and encryption. Its coming from the Windows-world and therefore lacking support for a number of features of Unix Systems.
p7zip is a port of 7za.exe for POSIX systems like Unix (Linux, Solaris, OpenBSD, FreeBSD, Cygwin, AIX, ...), MacOS X and BeOS.
J7zip is a JAVA port of 7zr.exe