る-Verb (Dictionary)る + ます (Ichidan Verbs)

る-Verbs are verbs that have only one possible conjugation pattern. This pattern simply involves the replacement of る with another construction, depending on the tense of the verb (past, negative, etc). This conjugation pattern that only includes る is where the Japanese name Ichidan (一段(いちだん)) 'one-level' comes from.

In these examples, we can see that る is removed from the verb to create the polite ます form. The る form of the verb is often called 'plain form', 'casual form', or 'short form', depending on the source/book.
All る-Verbs end in the kana る, there are no exceptions to this rule.

Caution

While all る-Verbs end in る, not all verbs that end in る are る-Verbs. These verbs can be identified by the る changing, rather than being removed when conjugated.

As the る in 取(と)る becomes り in 取(と)ります, we can determine immediately that it is not a る-Verb, but actually a う-Verb.