The Zealot is the Protoss base unit. However, it is more powerful than its Terran and Zerg counterparts, which also makes up for its higher price. You can rush an enemy using Zealots quickly, since you only need a Gateway to build them, and you can have 8-12 Zealots before they can create enough Zerglings or Marines to effectively stop you. However, they are vulnerable to air attacks, so keep some anti-air units close by later on in the game (Scouts work well for this).