Modal verbs are auxiliary verbs (also known as "helping verbs"). Normally modal verbs cannot work alone and must work with a main verb. The so-called "semi-modals" work partly like modals and partly like main verbs.
can, could, will, would, shall, should, may, might
People sometimes say that there is no difference between shall and will, or even that today nobody uses shall (except in offers such as Shall I call a taxi?). They say the same thing about should, but it's not really true.