Sell is a verb and sale is a noun:

I'm going to sell my car and buy a new one.

She's selling bottles of water at the football game.

Yesterday I sold all of my old college textbooks on the internet.

The bookstore is having a Christmas sale - everything is 20% off.

(20% off = 20% discount)

The sales of our new product are slowly increasing.

This watch is normally $100, but it's on sale for $70.

(on sale = there is a discount)

There's also a pronunciation difference between sale and sell.

Forever Young Naturally Exercise

Youthful Spirit Do something daring

Every week do something a bit daring: run through the backyard naked, start a conversation with a stranger on a train, go skinny dipping, phone in sick, wear a shorter skirt, book a surfing weekend or a parachute jump, call the person you lust after and ask them out to dinner. Enjoy the new you.