The Protoss have excellent ground units that prevent against any early game Reaper rush. The Colossus and Immortal are very robust when it comes to defending your base or initiating attacks. However, these units are not capable of fighting off air assaults.
After the first rush has been squelched, prepare for a Marine and Marauder invasion by your Terran opponent. These two units are relatively cheap and provide a solid "ball" of firepower that should be countered with Charge Zealots, Sentries, and Colossi. More specifically, Sentries provide a unique Force Field capability that can "break up" an M&M offensive. If performed correctly, you should have the advantage to pick off Terran ground units that are caught within your barricade. NOTE: this should be used at the foot of a ramp leading to your camp.
Protoss vs. Terran Unit Match-Ups
Finally, here are some essential Protoss vs. Terran unit match-ups that will equalize the competition or place you at an advantage during in-game battle: