Riding whips and crops
A whip or crop is a tool to reinforce your natural aids, i.e. your leg aids, seat and voice. A riding whip or riding crop is an extension of your arm to help your horse move forwards. A whip should never be abused to not desensitive the horse.
Crops are relatively short and mostly used in show jumping to reinforce the aid to jump. Riding whips are longer and are used to tap the hind of the horse rather than the neck.
It's important to only use a whip when truly needed. Don't rely on the use of a whip, always use your natural aids first and only reinforce your aids with a crack of the whip when there is no or little response.