The mature cactus plant will produce flower buds in the spring. These buds will develop into flowers in a matter of time, some a few days and some a few weeks. Often these flowers will open and close with daylight.
Bees, bats and birds will come to the highly colorful flowers of the cactus to eat the pollen and nectar. As they travel from cactus to cactus, the pollen will be carried to the anthers and the flowers will be pollinated just the same way as other fruiting plants.
The pollinated flower will then have a swelling at its base that will develop into a fruit. On most cacti, this fruit is brightly colored and sweet. Within the center of the fruit, the seeds are ripening.
An animal or bird is sure to find the fruit of the cacti, even if it is only after it drops to the ground. Birds tend to digest the seed while the rodents and larger mammals will just pass it though their digestive tract and deposit it elsewhere. A few of the deposited seeds will germinate and sprout into new plants.
The little seedling will slowly grow--sometimes only an inch or more a year, slowly maturing and growing until it is large enough to flower and start the life cycle all over again.