All those games share the same source (roleplaying), but occupy different niches. Diablo is a dungeon crawler, a roleplaying game where story and narrative arcs are less present, in favor of more combat. Skyrim is an action RPG, foregoing the most tactical aspects for some intense action, making it blends with the FPS genre. Baldur's Gate is a CRPG (for classic rpg). It focuses on characters development and story. So, compared to the other two, expect less intense fighting (for the record, it's turn based), more thinking, and way more story. What will feel familiar, though, is characters getting xp to level up and unlock new powers.