In the Marvel Universe, Hawkeye refers to two people: Clint Barton the Avenger, and Kate Bishop the Young Avenger. Both use bows and arrows, but Kate is skilled in other weapons while Clint is more well-establshed as a character.
Daredevil is Matt Murdock, a blind lawyer with a kind of extrasensory radar that helps him fight crime.
Bullseye is a psychotic killer, one of Daredevil's long-standing enemies. He was once responsible for the death of Daredevil's girlfriend, the reformed assassin Elektra. Fortunately (or unfortunately as the case may be), she was popular enough to be resurrected, but it was Bullseye's major claim to fame.
- Further undoing the confusion:
You remember that, before the Secret Invasion, Bullseye was a member of the Thunderbolts, right? And, after the Invasion, Norman Osborn decided to rechristen the team as the Dark Avengers, right? So, the idea of the Dark Avengers is to have Osborn and his cronies pass off as heroes (even using hero names) while secretly taking over the world with his cabal (Emma Frost, Namor, Dr. Doom, Loki and the Hood). And, in these Dark Avengers, since Bullseye is the expert marksman of the team, he was given the identity of Hawkeye, complete with the classic costume.
Clint Barton, for his part, has for quite some time (shortly after the whole House of M mess brought him back to life) "inherited" the Ronin identity from Echo, having also given Young Avenger Kate Bishop his blessing to use the name Hawkeye for herself.
As for the Daredevil part, Bullseye had once taken on his identity when Matt had to leave New York and then, he started playing Robin Hood (kept the proportions). However, the whole Daredevil stuff ended up going to his head, and he started thinking he was Daredevil until the real Daredevil (dressed up as Bullseye) came back and treated him to a good old knuckle sandwich, and things returned to normal (kept the proportions).