what is Triple DES?

Triple DES uses three different keys and three executions of DES algorithm.

The algorithm is
Cipher text = E k3 [D k2 [E k1 [Plain Text]]]
where E k [X] = DES Encryption of X using key K
and D k [X] = DES Decryption of X using key K

Remember that in DES Decryption of encrypted plain text with a different
key is almost same as another encryption. This is true as encryption and
decryption use the same algorithm.

To decrypt cipher text we reverse the operations.
Plain text = D k1 [E k2 [D k3 [Cipher Text]]]

Block Diagram of Triple DES: 

 Using DES thrice is equivalent to having a DES key length of 168 bits.
Brute force method to break triple DES with 106 decrypts per micro second
will take 5.9 X 10 30 years! Even at 1012 fold increase in computer speed
will make triple DES secure against brute force attacks to break code
The only reason D is used as middle step in triple DES is to allow decryption
of data encrypted using single DES hardware. In this case K3=K2=K1
(Single key used) (See block diagram)

Triple DES will be quite popular for a foreseeable future as it is very secure,
can be released by simple hardware. Triple DES has two disadvantages
1. It is slow to implement in software
2. It uses 64 bit blocks.
Thus new standards were explored.