[csande17]

I definitely remember only learning about two groups, personally. Is the third group, like, "irregular verbs", or do some books teach "the -suru verbs" as the third group (instead of "suru" being a single irregular verb that you can attach nouns to)?

[charcircuit]

The three groups taught to foreigners are:

- u verbs / Group 1 verbs / 五段活用

- ru verbs / Group 2 verbs / 一段活用

- Irregular verbs / Group 3 verbs / 不規則動詞

>or do some books teach "the -suru verbs" as the third group (instead of "suru" being a single irregular verb that you can attach nouns to)?

In books for native speakers instead of having irregular verbs they have:

- サ行変格活用 which are the suru and noun + する verbs

- カ行変格活用 which are the kuru verbs.