I keep reading that I should use a "waterproof membrane" for my bathroom floor (which has a tub/shower). But they never mention what could constitute a "waterproof membrane". What can I use for this? Is it the same as a "decoupling" material like Ditra?

I just know that if I walk into Home Depot asking for a "waterproof membrane" they'll point me toward the tarps.

If you are actually doing a shower stall home stores should carry this in the tile or flooring section it's a very thick either vinyl or pvc flexible membrane specifically for doing shower floors.
IF you are covering just the floor surface with tile and have a wood substrate at least 3/4 plywood then just use a good thin set and ceramic tile backer board. If I have missed anything please let me know.

A membrane and detra is 2 different products, detar being much more expensive but much more reliable. You can get a membrane from a plumming store, or possibly a roofing company, the membrane looks like an intertube. Black, its just rubber

As far as Lowe's or home depot, you need to ask in the tile section