According to the BBC, cacti have widespread, hardy root systems that collect as much water as possible in the area. Their enlarged stems can also store water and are fully capable of photosynthesis. A waxy, tough substance called the cuticle covers the plant and protects from dehydration.
The famous spines of a cactus are an adaptation which minimizes the surface area when compared to the more ubiquitous leaves of other plants. These spines also protect the cactus from the trespasses of herbivores. Encyclopedia Britannica states that cacti are flowering plants, with more than 2,000 species spread across North and South America.