Logically there is no qualitative difference between a lemma and a theorem. They are both statements whose value is either true or false. However, a lemma is seen more as a stepping-stone than a theorem in itself (and frequently takes a lot more work to prove than the theorem to which it leads).
Some lemmas are famous enough to be named after the mathematician who proved them (for example: Abel's Lemma and Urysohn's Lemma), but they are still categorised as second-class citizens in the aristocracy of mathematics.
The plural of lemma is either lemmas or lemmata.