The yeast cake/slurry at the bottom is a mixture of several things, but it contains a lot of live yeast.
Don't use a whole yeast cake for an average brew. Use the mrmalty calculator[/url:2vyj45ji] to see how much to use, and it will normally get going quicker than the dried stuff as it has already been hydrated.
You can put the slurry into one or more sterilised jars and they'll keep for quite some time in the fridge.