Jellies are plastic. Crocs are not. They are kind of like a sponge only non-poreous. Their Web site says, "Ventilated so dirt and air pass through but made of space-age material so they are odorless and virtually weightless. Ergonomic and orthotic, Crocs mold to your feet." and
"1.) Really soft, super comfortable, molds to your feet
2.) Barely there, weighing only 6 ounces
3.) Vented so air passes through, keeping feet cool
4.) Non-marking slip-resistant soles *
5.) Bacteria and odor resistant
6.) Ultra-hip Italian styling
7.) Port holes allow water and sand to pass through
8.) Can be sterilized in water and bleach
9.) Easy maintenance, just wipe clean
10.) Orthotic molded foot bed for ultimate comfort and support"
I have to say I don't necessarily agree with 6. Re: 4 if you wear them down, they eventually get slick on wet floors.
I like them because they truly are the most comfortable shoes I own. I wish there was a dressy version that I could wear to work and such though. You know what the material reminds me of? That yellowish stuff on display at Lowes and Home Depot that's used to squirt in for insulation. Foamy soft. Not like rubber exactly. Not like plastic AT ALL. They were my running shoes a few years ago when I lived in Hawaii, jogging up and down very steep roads (highway to ocean) and I even ran along the water line, getting salt water and sand in them and continuing to run as they dried out. Yes they made a squeek, but even drying salt didn't irritate my feet.
Anyway, hope that helps. Summing up my opinion: comfy as all heck, not too spendy, not too pretty

BTW I have the Beach model as mine are short fat feet.