ok theres Silicone cases which are really grippy often flimsy and fit phones kind of loose, they offer really good protection but get stuck in your pocket and attract a **** load of lint, and also because they dont fit tight they let a lot of lint and dirt in between the case and phone
theres hard plastic cases. they come with the grippy matte(rubbery) like material that a lot of like HTC phone use on their back covers, and they also come painted with all sorts of designs and in solid colors and also come in clear, and transparent colors(like smoke and all the colors of the rainbow), these protect your phone very well but the downfall is dirt and sharp edges.
most of these cases are a bit of a bitch to get on and off and most people leave them on for a long long time only to find out that dirt and lint got in between the case and phone and left horrible rub marks all over the phone, not to mention that while flexing and moving the case tends to scratch the contact points on the phone also.
then theres my favorite Gel/Jelly/Crystal/TPU all of which are the same they are a hard rubberish case that in most cases are easy to remove and put on and fit the phone like a glove (not leaving a lot of room for dust and dirt to get in) not meaning that dirt doesn't get in but its minimal when compared to the other two types of cases. they protect your phone just as well as any other case, have moderate grip (not too grippy like the Silicone and not too little grip like the hard cases) i compare the feel to kind of like the invisible shield "kind of grip" they come in solid colors and in clear tints
Ive bought all of these cases before on multiple phones (G1, Mytouch, Nexus, HD 2) and I have given all of these a chance and no matter how many times i buy a hard case or a silicone case i always end up with a Gel/TPU case, they fit great add the least amount of bulk out of the three and have saved all of my phones from concrete and tile drops multiple times (almost **** my pants when i did it with my Nexus and HD2 though
with all these cases your going to have to remove the case every once in a while to get the dust out that gets in between the case and phone (unless you like scratches on your phone) thats the common downside to all of these cases but in return you have the security that you wont crack or damage your phone and it will last you