This article is from the PGP FAQ, by Jeff Licquia firstname.lastname@example.org with numerous contributions by others.
You would think that's an easy question to answer!
At the moment, there are four different "current" versions of PGP.
All of these are derived, more or less, from a common source base: PGP
2.3a, the last "guerillaware" version of PGP. Negotiations to make
PGP legal and "legitimate" have resulted in the differing versions
available; all of them, for the most part, are approximately
equivalent in functionality, and they can all work with each other in
MIT PGP 2.6.2 is the current "official" freeware version. It has been
developed both with Phil Zimmermann's approval and active involvement.
It contains several bug fixes and enhancements over 2.3a, and it
avoids the patent question surrounding other versions of PGP by using
the RSAREF library for some of its functions. This library was
developed by RSA Data Security, Inc., and is (basically) free for
noncommercial use. As part of MIT's agreement with RSADSI, all
versions of MIT PGP generate encrypted messages that cannot be
decrypted with PGP 2.3a or previous versions.
ViaCrypt PGP 2.7.1 is the current "official" commercial version. It
is available from ViaCrypt, a company out of Arizona, and also has
Phil's approval and involvement. See below for details on this
PGP 2.6.2i ("international") is a version of PGP developed from the
source code of MIT PGP, which was exported illegally from the United
States at some point. Basically, it is MIT PGP 2.6.2, but it uses the
old encryption routines from PGP 2.3a; these routines perform better
than RSAREF and in addition do not have the usage restrictions in the
RSAREF copyright license. It also contains some fixes for bugs
discovered since the release of MIT PGP 2.6.2.
PGP 2.6ui ("unofficial international") is PGP 2.3a with minor
modifications made so it can decrypt files encrypted with MIT PGP. It
does not contain any of the MIT fixes and improvements; it does,
however, have other improvements, most notably in the Macintosh