Lettuce go to the salad bar, we're starving!
A pun is a play on words. The sound or spelling of the words might be similar, sure, but the meanings are very different. Hilarity ensues. Or, more often than not, groans.
For an example of how puns work in literature, see our discussion of the character of The Happy Medium in Madeleine L'Engle's A Wrinkle in Time. Or read just about any page of Shakespeare. Big Willy was notorious for his puns, the dirtier the better.