DES uses a short 56 bit key, which was vulnerable to brute force attacks. 3DES uses 168 bit keys (56x3), and performs the encryption like so:
- Encrypt clear text using the 1st 56 bits of the key, produce output1
- Decrypt output1 using the 2nd 56 bits of the key, produce output2
- Encrypt output2 using the 3rd 56 bits of they, produce encrypted text.
Her's a reference: http://en.wikipedia.org/wiki/Triple_DES