Pure alkali. It needs an acid in the recipe — buttermilk, yoghurt, treacle, cocoa — to produce any lift at all, and without one it leaves a soapy, metallic taste. That dependence is the whole difference from baking powder.
Sourcing
The baking aisle sells small expensive tubs; the cleaning aisle and pharmacies sell the identical thing in large cheap bags.
Quality cues
Should fizz hard when a spoonful meets vinegar. It is very stable, so it rarely fails.
Substitutions
Baking powder (use about three times as much)
Storage
Airtight and dry — it absorbs both moisture and odours, which is why an open box works as a fridge deodoriser.