Aes is considered more secure than des and triple des because?

Well because it's the newest. DES has been around since 1973 but wasn't formally a federal standard until 1976. It had to recertify every 5 years, the probem with this was....No company's came forth to go against the standard. So 30 years go by and tons of people have found ways to decipher DES. Weak keys are a major problem, and there are only a certain amount of keys allowed due to it's bit size. 2^56 keys available. It's susceptable to brute force attacks. 3DES goes through 3 rounds of DES, but it still has the same weaknesses. After 30 years of the standard, NIST called for a new standard the AES, it went with a contest design. There were 5 finalist. MARS by IBM, RC6 by RSA, Rijndael by John Daemen and Vincent Rigmen, Serpent by Ross Anderson, Eli Biham, and Lars Knudsen, and Twofish by Bruce Schneier, John Kelsey, Doug Whiting, David Wagner, Chris Hall, and Niels Fergson. The winner was Rigndael which was chosen in 2000.

The competition called for a block cipher using symmetric Key cryptography. It also had to support key sizes of 128, 192 and 256 bits. For big and small devices. It standardizes at 128 bits blocks.

the size of the key depends on the number of rounds of encrpytion.

Wikipedia it to find out more about how the new standard works. Since it has 3 different sizes, brute force attack with current systems would take a really long time. But as technology grows so will the need for an even stronger crypto system. 3AES may be the next standard :)