This article is from the PGP FAQ, by Jeff Licquia email@example.com with numerous contributions by others.
Currently, the best attack possible on PGP is a dictionary attack on
the pass phrase. This is an attack where a program picks words out of
a dictionary and strings them together in different ways in an attempt
to guess your pass phrase.
This is why picking a strong pass phrase is so important. Many of
these cracker programs are very sophisticated and can take advantage
of language idioms, popular phrases, and rules of grammar in building
their guesses. Single-word "phrases", proper names (especially famous
ones), or famous quotes are almost always crackable by a program with
any "smarts" in it at all.