"ir" verbs usually indicates on movement (going somewhere). This verb can usually also stand alone and can act a singular word.
example: "Ma', puedo ir a la playa con mis amigos este sabado?"
The girl is asking her mom if she can go to the beach with her friends on Saturday.
"er" usually indicates present-tense. It cannot stand alone as a singular word the way "ir" can.
example: Saber (to know)
Finally "ar" is a less used verb in spanish. It also cannot stand alone as a word. "ar" and "er" are very simaler verbs, since they both indicate to do something.
