A good trilogy of articles by Peter van der Linden on encryption.
Part 1, describes how public key encryption works and where to get the GnuPG encryption software. Part 2 walks through some examples of file encryption/decryption, and Part 3 explains how to send and read encrypted e-mail
Lesson in Encryption, Part 1
Lesson in Encryption, Part 2
Lesson in Encryption, Part 3
– Suramya