There are a variety of surfaces found in the game.
Lava - it looks like a dark red surface, covered in cracks that are bright red. If you touch the lava, you'll be launched away from it, causing you to die. However, you can climb certain lava surfaces if you slowly touch it. You can even jump on lava without dying by going on top of it while pressing W or (^). This trick works best with lava that's faced horizontally.
Water - water is water. It has similar properties to water in real life. If your mouse jumps on water, it will float. You can swim in the water using your regular controls (WASD/(<^>down). You will sink in water if you have cheese with you. If you try to stay put in water, you will eventually drift away. Water in transformice doesn't free flow/follow the shape of it's container like in Minecraft. That means you can even find water floating in mid air, don't be surprised.
Wood - wood is usually safe to touch. You can wall jump it to climb it, which means you can't use the (W,D/^>) keys to climb it.
Ice - Ice looks whitish-blue and is solid, which means mice can walk on it. It's also slippery. If you try to stay put on ice, you will eventually drift away.
Trampoline/Tramp - This surface is purple and bouncy. Mice can bounce on it to go to higher places.
Cloud - This surface looks translucent white. Mice can't touch it (only on certain maps the cloud is touchable.). A shaman item can't pass through a cloud.