Much like your standard action/RPG, levelling up will get you skill points as well as stat points to put where you want. Each of D&D Heroes' four playable characters (Elf Sorceress, Human Warrior, Dwarf Cleric, Halfling Rogue) has a pretty wide range of skills, most of which can be upgraded multiple times. This is probably the source of most of the game's replayability, as there are a ton of combinations available for each character. By the time you get through the game twice, though, you'll have enough points to fill everything out, so that's a good time to try out another of the four characters.