An Acronym for Microsoft Point-to-Point Encryption.
It is a protocol designed for transfering encrypted datagrams over point-to-point links using encryption keys varying in length from 40 bits to 128 bits. See 
RFC:3078 and RFC:3079 for details.
If you want encrypt data when talking to Windows clients over a PPTP link, you will need MPPE support in your Kernel and pppd(8) (it comes built in in PPP since 2.4.2). Patches are available from 
http://public.planetmirror.com/pub/mppe/ and http://pserver.samba.org/cgi-bin/cvsweb/ppp/linux/mppe/.
See also:
6 pages link to MPPE:
