a joke exploiting the different possible meanings of a word

Puns are most commonly used for humorous effect.

Puns can also be non humorous as well. Puns are most commonly used to add humor to a piece of literature or a book.

Examples of puns:

The life of a patient of hypertension is always at steak.
Why do we still have troops in Germany? To keep the Russians in Czech.
A horse is a very stable animal.
Time flies like an arrow. Fruit flies like a banana.
An elephant’s opinion carries a lot of weight.
What is the difference between a conductor and a teacher? The conductor minds the train and a teacher trains the mind.