In some Japanese textbooks, the term -ru verb (or ru-verb) means the same thing as an ichidan verb. Such verbs always end in ~ru ～る in their nonpast form (or dictionary form, shuushikei 終止形), however, they aren't the only ones to do so: godan verbs (or -u verbs) can also end in ~ru in nonpast form, which means not every verb that ends in ~ru is a "ru verb," making the term extremely confusing for lots of people trying to learn Japanese.(Steffanick, 2010:20)
Grammar
See the article about ichidan verbs for details on grammar and conjugation. This article is mainly about the term -ru verb itself.
Explanation of the Term
The reason why -ru verbs are called -ru verbs even though not all ~ru-ending verbs are -ru verbs is because the term refers to the morphology of the verb: when a -ru verb is conjugated, everything before the ~ru syllable stays the same (i.e. the stem is before ~ru), and only the ~ru gets replaced by a new suffix.
By contrast, with -u verbs, the ~u vowel is replaced by another vowel (the stem ends at the consonant before the vowel). For example:
| -ru verb | -u verb | |
|---|---|---|
| Nonpast form | ki-ru 着る Will wear. |
kir-u 切る Will sever. |
| Past form | ki-ta 着た Wore. |
*kir-ita (merges by sokuonbin 促音便.) kitta 切った Severed. |
| Negative form | ki-nai 着ない Won't wear. |
kir-anai 切らない Won't sever. |
| Volitional form | ki-you 着よう Let's wear. |
kir-ou 切ろう Let's cut. |
So -ru verb means "the -ru ending gets replaced"-verb, and -u verb means "the -u ending gets replaced"-verb.
To make matters more confusing, suru する and kuru 来る are irregular verbs that end in ~ru (sahen サ変 and kahen カ変 conjugation, respectively). These also aren't called -ru verbs, because they don't follow the ichidan conjugation, e.g. their past forms are shita した and kita きた, respectively, so it's not only the ~ru that changes..
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