Perhaps you're thinking of:
* A poem with a single rhyming couplet: Two lines that rhyme, like:
> The cat sat on the mat,
> And watched the little rat.
* A poem with all lines ending in the same rhyme: This is a bit more common, and can be quite effective:
> The wind howls in the trees,
> A chill runs through my knees,
> The night is dark and cold,
> A story yet untold.
Let me know if you have any other questions!