For hobby gardeners, I would simplify them as evergreen flowering plant which absorb water and nutrients from the air. They use their roots for anchoring only.
For More botanical details visit here tillandsia Wiki
These plants are trending among gardeners who are involved in-house plants, terrariums, indoor garden design and creating garden crafts using sea shells and bulbs. It makes them a perfect gardening gift for your family and friend.
Some inspirations from amazon (yay ! do not have pics handy as of now) which showcase the plants with different base like crystal, seashells etc.