Also known as near rhyme (or half rhyme, weak rhyme, or imperfect rhyme), these rhymes aren't quite… rhymes. They come close, but alas, no cigar. The words "dear" and "door" form a slant rhyme. The words sound similar, but they aren't close enough to make a full rhyme.
You'll see these guys all over poetry. Really, they're just about everywhere you look. But poets aren't just tossing these in because they were too lazy to get out the rhyming dictionary and find a perfect rhyme. No, they use 'em for a reason, and their reasons are as varied as the poems themselves.