Beat Hazard Ultra is DLC for Beat Hazard, acting as an expansion pack which adds to the core gameplay. Like the base game, Ultra is a top-down procedurally generated shooter which takes your music files and generates waves of enemies that synchronize to the music. The purpose is to get a high score, while using various power ups (volume, multipliers, points, etc) to facilitate this score. As you play, there are a series of upgrades which can modify your game to allow for different challenges and playstyles.
Ultra adds some new weapons, enemy types, achievements, online play (both co-op and vs), a perk system, boss rush modes, leaderboards, and a few other small bits - it effectively completely reworks and adds to the game, while staying fundamentally the same at its core. If you enjoyed Beat Hazard, this is absolutely a worthwhile purchase.