The Triple-DES variant was developed after it became clear that DES by itself was too easy to crack. It uses three 56-bit DES keys, giving a total key length of 168 bits. Encryption using Triple-DES is simply
encryption using DES with the first 56-bit key
decryption using DES with the second 56-bit key
encryption using DES with the third 56-bit key
Because Triple-DES applies the DES algorithm three times (hence the name), Triple-DES takes three times as long as standard DES. Decryption using Triple-DES is the same as the encryption, except it is executed in reverse.