There are Four type of symmetric encryption algorithms in .net.

DES 

The Data Encryption Standard (DES) was developed by an IBM team around 1974 and adopted as a national standard in 1977. DES encrypts and decrypts data in 64-bit blocks, using a 64-bit key. Although the input key for DES is 64 bits long, the actual key used by DES is only 56 bits in length. The least significant (right-most) bit in each byte is a parity bit, and should be set so that there are always an odd number of 1s in every byte. These parity bits are ignored, so only the seven most significant bits of each byte are used, resulting in a key length of 56 bits. 

Triple DES 

Triple DES is three times slower than regular DES but can be billions of times more secure if used properly. Triple DES is simply another mode of DES operation. It takes three 64-bit keys, for an overall key length of 192 bits. The procedure for encryption is exactly the same as regular DES, but it is repeated three times, hence the name Triple DES. The data is encrypted with the first key, encrypted with the second key, and finally encrypted again with the third key. Triple DES enjoys much wider use than DES because DES is so easy to break with today's rapidly advancing technology. 

RC2

RC2 (Rivest Cipher) was designed by Ron Rivest as a replacement for DES and boasts a 3 times speed increase over DES. The input and output block sizes are 64 bits each. The key size is variable, from one byte up to 128 bytes, although the current implementation uses eight bytes. The algorithm is designed to be easy to implement on 16-bit microprocessors.

Rijndael

The National Institute of Standards and Technology (NIST) officially announced that Rijndael, designed by Joan Daemen and Vincent Rijmen, would be the new Advanced Encryption Standard.

The Advanced Encryption Standard (AES) is the current encryption standard, intended to be used by  the U.S. Government organisations to protect sensitive (and even secret and top secret) information. It is also becoming a global standard for commercial software and hardware that use encryption. It is a block cipher which uses 128-bit, 192-bit or 256-bit keys. Rijndael is very secure and has no known weaknesses.