Placing a Cactus Fruit in the crafting grid gives you a Cactus Fruit Seed.
Cactus Fruits can be planted directly onto sand to create crops, or turned into seeds to plant onto tilled soil. However, Cactus Fruits may not be planted directly onto farmland, nor can seeds be planted on sand. (Whether or not a method works can vary per modpack.)
Once they're fully grown you can right-click them, harvest the Cactus Fruit and let them grow again. When harvested by right-click, the plant doesn't break, and yields 1-3 Cactus Fruits.
Shapeless Crafting Edit
Here are all the recipes using Cactus Fruits: