In tubs side by side they look much the same. They start from completely different places, though, which decides exactly where one can stand in for the other.
They start from different places
Ricotta is made from the whey left over after other cheese, heated again to catch the protein still in it — the name means re-cooked. Cottage cheese is made by setting milk itself into curds. One comes from a by-product and one comes straight from milk, so the resemblance between them is close to a coincidence. That single fact produces every difference below.
Alike in being white and curdy:
You can tell by looking
Ricotta's curds are fine, so it holds together smoothly and sits in very little liquid. Cottage cheese has large curds you can feel between the teeth, with cream filling the gaps and visible liquid moving separately. Tilt the tub and the answer is immediate. Cottage cheese is also sold in large-curd and small-curd versions: the large ones chew more distinctly, the small ones carry more moisture.
Flavour and nutrition differ too
Ricotta is faintly sweet and barely salted. Being made from whey it is low in fat and very mild, so whatever you put with it largely decides the flavour. Cottage cheese carries both a light sourness and salt: the curds are rinsed to remove salt and then dressed with cream again, so a bland curd and a rich cream arrive in the same spoonful. If protein is the point, cottage cheese delivers more of it per gram of fat.
Can you swap them?
It depends on the job. Spooned onto bread or through a salad, either works. In a lasagne filling or a dessert — anywhere it has to bind smoothly — cottage cheese does not, because the coarse curds and separate liquid weep in the oven. Draining it in a sieve and blending it smooth gets you close, but that is an extra step. Going the other way, ricotta in a cottage cheese's place loses the chew and reads as flat.
What to look for
With cottage cheese, check curd size and fat. Large curds are set with rennet, so they are less sour and a little sweeter; small curds are tangier. Four percent fat has cream added and is thick, while one to two percent is wetter and leaner — and two percent is the safe middle if you are unsure. With ricotta, versions made from sheep or buffalo whey are richer than cow's, so glance at the ingredients if you plan to eat it by the spoon.