Worshipper – “Agral’s Decision” [Campaign 2, Mission 5]

What are patches? And what is an expansion pack? I have the Hammers of Fate disc, but I haven't played it yet. Am I missing something?

Hammers of Fate is the expansion pack. It is installed after the main game is installed. You play it separately from the main game.
Patches are updates to the game. They fix the balance and gameplay mechanics. You need versions 1.4, 1.41, and 1.5.