Zeal is the standard attack skill of most Paladins. It is quite powerful in its own right, as if hits much faster than regular attacks, something which doesn't appear in the skill statistics. A Paladin using this skill is referred to as a Zealer or Zealadin (not to be confused with Zakarum Zealot, a monster in Act III).
Zeal can often attack extremely fast. This can create some dangerous situations, which prior to Patch 1.13 most notably happened in the Chaos Sanctuary. If an Oblivion Knight were to cast Iron Maiden (which they no longer do) while the Paladin was in mid-attack, it was possible for the Paladin to instantly kill himself.
A noble Paladin, fervent in his dedication to righteousness, can draw upon the spirit of that dedication to perform seemingly impossible tasks. When surrounded by his enemies, a Paladin versed in this skill sets upon his adversaries with the zealous fervor of many times his number. 
↑Diablo II Manual, page 79. Blizzard Entertainment, 2000.