You gain xp (experience) each time you attack. Specifically:

If you don't attack in a round, you get 0 xp.
If you miss, you get 1 xp.
If you hit, you get 10 xp, -1 for each level you are above your opponent, +1 for each level you are below your opponent. Minimum of 1 xp for hitting.
If you hit and kill, you get 30 xp, -3 for each level you are above your opponent, +3 for each level you are below your opponent. Minimum of 3 xp for hitting and killing.
Tinking (dealing 0 damage on a hit) has no effect on the amount of experience you get: You get the same amount of experience for tinking as you would for hitting and doing damage.