There is dry yeast and wet yeast. Wet yeast like those from White Labs and WYeast have more variety to choose from and is stored in liquid form. Dry yeast comes in a packet and most have a billion or more yeast cells than their liquid counterparts. Dry costs less.
Dry yeast keeps much longer. The packets you buy are dated, and if stored in foil wrappers in a cool, dry place, well protected from air, the yeast will be good for use at least as long as the date indicates.
Here’s a tip to save you tons of $$$ on your dry yeast: eBay.
I use yeast for making bread, but you’ll find the same kind of deals for your favorite dry brewing yeast there, too. I bought two 1-pound units of Fleischmann’s Instant Dry Yeast in foil packaging for a total of $12.94 (including shipping) on eBay the first week of this month. The same kind of pricing is there for active dry yeast, if you use that.
It’s sooooooooo much cheaper to buy it like this than to pay grocery store prices!
The packages that I just bought are dated March 2013. But no matter how quickly or slowly you use it, your yeast will keep almost indefinitely if you store it in the freezer.