The difference between suet and tallow? I just received a couple kilograms worth of raw lamb suet from a butcher free of charge. My understanding is that the fat surrounding the kidney (which this is) is suet and that rendered fat (starting from suet or otherwise) is tallow.

Tallow, to my understanding, has a much longer life that this raw suet that I have. I'm planning to render the suet down but was wondering if I could still use in recipes that call for suet after it has been rendered? Is it better in the raw state or does it make no difference?

My main usage would probably be in pastries and tortillas.
If you're making tortillas and pastry dough, you'd want to use the rendered suet = tallow.

If you're making something like a traditional English Christmas pudding or a spotted dick, you'd want to use the un-rendered (but cleaned and shredded) suet.

Either one would last pretty much indefinitely in the freezer. Suet would have a shorter lifespan at room temperature (since it's still got bits of meat attached).