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NIST defined 3DES or Triple DES in 1999.
3DES uses three stages of DES so it is much more secure and suffices for most applications currently.
DES is a block cipher - i.e. it acts on a fixed-length block of plaintext and converts it into a block of cipher text of the same size by using the secret key.
In DES, the block size for plaintext is 64 bits.
The length of the key is also 64 bits but 8 bits are used for parity.
Hence the effective key length is only 56 bits.
In 3DES, we apply 3 stages of DES with a separate key for each stage.
So the key length in 3DES is 168 bits.
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