Well, they're both games where you follow a linear story through different chapters, with a lead character you created by picking a class, atributes and physical characteristics. Furthermore, you go through both games picking up a motley crew of adventurers, with whom you talk to further each of the character's plots. Also, you go around doing quests, mostly of the fetch or kill variety which usually can be completed in either a good or evil fashion. Also, the combat follows a formula where you take the characters you picked to follow you plus the lead character to fight against all manner of beasties like ghouls, skeletons, spiders, orcs and many humans, who may be thieves, or bandits, or evil priests or whatever. It should be noted these fights are somewhat tactical and follow a set of arbitrary rules known as Dungeons & Dragon where the classes and attributes of the characters involved determine the outcome of battles. Finally, both games take place in the same fictional universe.
Completely different games. It's like comparing Civilization and Halo.