Poetry in stanzas of three lines with a regular rhyme scheme is called an "ABA" poem. If one line of the stanza does not contain the same number of syllables as the other two, the omitted line is usually the last. An ABBA poem has four stresses on the first line, three stresses on the second line, and two stresses on the third line. This is very common in English poetry, but not always apparent from reading poems because most languages including English have varying numbers of each sound and so some poets choose to vary the number of stresses to reflect this. For example, Robert Frost often uses alliteration and consonance to attract attention in his poems. The only requirement for a poem to be considered poetic is that it should give off a feeling of something beautiful or interesting. That's it! Any old thing you write down could be considered a poem if you feel like it can be called such.