1st conjugation group

Verbs in this group finish with "ER" e.g. manger, nager, jouer

2nd conjugation group

Verbs in this group finish with "IR" e.g. finir, salir, 

3rd congugation group

Verbs in this group are irregular and can finish in "RE", "OIR", "IR" or "ER". e.g. être, savoir, rire, aller. 

One way to differentiate verbs from the second group and the 3rd group is by conjugating them at the Participe Passé tense. If it finishes in "issant", it belongs to the second group.  For instance "salir" would be "il est salissant" once conjugated, making it a 2nd group verb and not a 3rd group one.