Shape or form is also important in a poem.  It helps the reader understand or react to the poem.

A traditional poem may give four lines and a break, four lines and a break, etc.  Each four lines may talk about something slightly different. That four-line pattern is the poem's shape or form. 

Modern poems can actually use “shape” as the most noticeable part of the poem itself.  Look at the poem that follows.  It is about a tree, but it also looks like a tree.