Slippery Blocks are created using the Freezer and are simply just as their title implies, slippery. When you walk on ice, you will glide across the block a little when you stop. These blocks do the same thing but can be even more intense in their slipperiness. Any normal block can be turned into a slippery block. Normal being defined as a non-tile entity with a normal block model. Stairs can also be changed into slippery blocks, but not slabs (this may change in future updates). Slippery blocks do not melt so feel free to use any light sources you wish.
Properties of certain blocks such as glowstone and redstone blocks do not work with slippery blocks, so they will not produce light or provide a redstone signal.
There are four tiers of slipperiness a Slippery Block can have: 1x, 2x, 3x, and 4x. With each tier, the block will become harder to come to a complete stop while moving atop of it. To increase a slippery block's slipperiness, simply place it back into the Freezer with some water and let it freeze.
As stated earlier, use the Freezer to craft these blocks. Place a normal block (non-tile entity with a normal block model and stairs work too) into the top slot and deposit at least 250 mB (1/4 bucket) of water into the freezer. Provide fuel for the Freezer (Frezarite Crystal or Ice) and let it freeze. Then to increase the slipperiness of the block, place it back into the top slot with another 250 mB of water.
Slippery Blocks are practical for a few things. One idea is to use the speed tunnel technique where you have a 2 high by 1 wide tunnel with slippery blocks for the floor. Then run and jump through the tunnel to travel at very fast speeds. The more slippery the block, the faster you will go. Since slippery blocks can look like any normal block, it can make the tunnel look any way you want.
Another use for them is traps. You can place tripwire on slippery blocks so this makes it practical for a trap. Simply make the slippery blocks look like the floor and place a few slippery blocks before the tripwire. The other player may see the tripwire ahead of time and try to stop, but the slippery blocks will make it harder for them to stop. I'll leave it up to you on to what you want the trap to do.
Get creative with these blocks and you can probably come up with a few great ideas on how to use them besides what I've mentioned here.