Swallows and swifts:
Two completely different families of birds.
Swifts are apodidae, "footless". Four toes turned forward, legs too short to walk, or groom, so they suffer from parasites. Very long wings, which they sometimes beat alternately, so they seem to rock as they fly. They have very short "arms", so their wings look like simple sickle blades. Look all dark. Often fly in groups around buildings, screaming.
Swallows are hirundines, with three UK species: swallow, and sand and house martins. The two martins have shorter tails than the swallow, but all three have white bellies.
Swallow and house martin: dark blue-black, white belly.
House martin: white rump.
Sand martin: brownish, no white rump. smaller, brown band across breast.
All three twitter in flight. Swallows and house martins make mud nests, sand martins use burrows.