Japanese verbs mostly fall into three conjugation categories, besides a small number of irregular verbs.
These three categories are called ichidan verbs, godan verbs, and suru verbs.
Ichidan (一段いちだん) verbs, also known as -ru verbs, always end in either -iru or -eru. The final る drops in many conjugations.
Godan (五段ごだん) verbs, also known as -u verbs, can end in any of the following letters: う・く・ぐ・す・つ・ぬ・ぶ・む・る. These verbs have different sound changes in the past tense and -te form depending on their final letter, and they are grouped according to this into different verb models for each possible final letter.
Suru (する) verbs end in the auxiliary verb する “do”, often formed by adding する after a noun to turn it into a verb, such as 勉べん強きょう “study (noun)”, 勉べん強きょうする “study (verb)”.
There are a handful of verbs in Japanese with irregular conjugations that do not fit into the previous categories.