There is one Guardian for each Element; Fire, Electricity, Ice and Earth. Though Wind is an element that exists in the universe, there is no guardian of that element. Whether there used to be, or shouldn't be, is unknown.
The Guardian of Fire
Ignitus is the master of Fire. He can do, or used to do, very powerful fire attacks, but, as it is shown in A New Beginning, doing a fury attack is difficult for him and it takes a lot more time for him to recover since he has grown older. Ignitus is Spyro's father figure and mentor in The Legend of Spyro series.
The Guardian of Ice
Cyril is the master of Ice. He is very arrogant and proud, and he and Volteer argue a lot and it's usually up to Ignitus to stop them.
The Guardian of Earth
Terrador is the master of Earth. He is second in command, and the most militant and most calm of the Guardians. Within the range of Attack Power, Terrador seems to be the strongest.
The Guardian of Electricity
Volteer is the master of Electricity. He talks a lot about nothing, usually using big words while doing so, making him seem very enegetic for his age. He and Cyril argue a lot and its usually up to Ignitus to stop them.