An ancient four line stanza or poem still common in all European languages. Quatrains are often standalone forms. This is actually a quite common form of poetic structure, not necessarily in a purely quatrain form. The most common form of rhyme structure is abab, a four line structure with two alternating rhyming components. This isn't a quatrain, but it operates on the same basis in poetic structures. Rhyming quatrains can be highly flexible, structured aaaa, aabb abab, baba, etc. They can also use multiple form rhymes, where a single line may use ab, which is repeated in the following line.
Examples of Quatrain: