From Destinypedia, the Destiny wiki
They are equipped with suits of powered armor that boost their strength and endurance. Titans utilize their strength and skill with firearms to kill their enemies.
Titans have long honed the Light of the Traveler, passing these skills to each new generation of warrior. Their strength gave birth to the City, and they have taken a blood oath to ensure it will always stand strong. The Wall surrounding the City was built by the first Titans, who also perished defending it. Steadfast and suresighted, Titans face any challenge head-on, serving as living, blunt force instruments of the Traveler's will.
All Guardians have certain abilities, some of which are granted by the Traveler. All Guardians have a vertical movement ability and a "grenade-type" ability whose effects vary by class. All these abilities with the exception of the double jump can be upgraded over time and can be used in competitive PvP.
List of appearances