What are group I and group II (also consonant and vowel) verbs?

There are two kinds of regular verb in Japanese. One kind, "consonant stem", "group I" or, in Japanese, godan verbs, conjugates such that the rightmost unchanging element is a consonant. For example, iku (to go) conjugates as ikeru, ikinasai, ikanai, ikou. The other group, "vowel stem", "group II", or, in Japanese, ichidan verbs, conjugates such that the rightmost unchanging element is a vowel. For example, taberu (to eat) conjugates as taberareru, tabenasai, tabenai, tabeyou.

Form Group I Group II
Dictionary form iku taberu
Negative ikanai tabenai
Imperative ikinasai tabenasai
Passive ikareru taberareru
Volitional ikou tabeyou

The vowel stem verbs all end in -iru or -eru. However, not all verbs that end in -iru or -eru are vowel stem verbs. The next answer (Which and verbs are actually group I (consonant-stem, godan) verbs?) gives some rules and a list of the odd ones out.

Some "consonant-stem" verbs like warau (笑う) actually have inflections which end in a verb.

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