This is from Wikipedia:
Powers and abilities
While Hawkeye has no superhuman powers (with the exception of the period when using Pym particles to become Goliath), he is a trained athlete and tactician, being at the very peak of human conditioning. The character is a grandmaster marksman, having been trained by the criminal archer Trick Shot, and has been a highly trained acrobat and aerialist since his days in the circus. Hawkeye has been thoroughly trained by Captain America in tactics, martial arts, and hand-to-hand combat. Hawkeye excels in the use of weapons, especially the bow and arrow, and carries a quiver containing a number of customized "trick-arrows" that use sonics, gas, explosives, and flares, among others. In his new role as Ronin, Barton shows great proficiency with the katana and other melee weapons.
Here's a link to trick arrows from Wikipedia. It has a list of the different trick arrows used by both Hawkeye and Green Arrow:
http://en.wikipedia.org/wiki/Trick_arrows