Reusing the yeast

OK, I've been reading the threads for some time. I am a newbie - 4 batches so far - and I have seen several threads about using the yeast cake after racking to secondary/bottles. First, is this the trub or is there some filtering? There seems to be other stuff in there than just yeast. Second, what is the benefit to reusing vs pitching new yeast?

Thanks folks

Yes the yeast cake is the trub. So yes...there is more then just yeast in there. Some people wash there yeast, you may want to search for that and see what you can find(https://www.homebrewtalk.com/wiki/index.php/Washing_yeast.) The adv. of pitching on a yeast cake is that you do not have to buy yeast, you dont have to clean the primary, its gets real violent during the second fermentation and is fun to watch. Disadv. would be, it is not all yeast, you have to use the same strain of yeast for two beers in a row....and i cant think of others right now..but im sure there are some more.
Give it a whirl one day....

Read up on harvesting yeast from a yeast cake. It'll be worth it. I haven't bought yeast since the end of winter.