Combat, or PvP, is when a player attacks another player. Combat performance is boosted by such things as weaponry (see below), buffs from fruit, stats, and other items (such as shields or battle standards). In general, players can only make two attack actions per turn, unless boosted by something such as prayer to the deity TR. Additionally, players automatically retaliate when attacked, except in the case of ranged attacks. A retaliation is roughly equivalent to a normal attack, except there is no retaliation response to it (duh, or there would be an infinite chain until death).
Weaponry increases the damage per attack, and a few weapons attack multiple times. Notably, it is also possible to throw items to deal damage instead of directly attacking them, but the target will keep the item (and may use it to throw against The Wall to regenerate).