Many companies manufacture them and call them different names but they are all basically the same thing. They can come in different shapes like "crystals" which are jagged or the ones we got are "beads" which are round. They come in a variety of colors, too!
We got our at Michael's. Two packs filled her sensory tub (which I now put on an upside down crate to form more of a table set up) which cost less than $6.
They looked like this when we bought them, in clear and purple.
If you choose colored water beads I recommend rinsing the beads first. Our bag had a lot of dye powder in the bottom. I poured the bags right into her bin and added water. It looked like grape juice! I had to wait until the beads absorbed enough water that I could rinse them in a colander without them falling through into the sink. DO NOT let these fall into your drains obviously.
Four to six hours later the beads had absorbed a LOT of water (4 liters per bag, max) and were ready for play. My husband said these are the most irresistible material I've put in the sensory bin yet! He enjoyed exploring them with our daughter.
I put the lid on the bin when we aren't playing and add water every few days. They can get moldy after awhile so check them before playing! When you are done with them you can mix them into your potted plants.