Flightless birds are birds that through evolution lost the ability to fly. Two key differences between flying and flightless birds are the smaller wing bones of flightless birds and the absent (or greatly reduced) keel on their breastbone. The keel anchors muscles needed for wing movement. Flightless birds also have more feathers than flying birds.
There are over 60 extant species, and here is a few of them -
Elephant birds (extinct)
Great Auk (extinct)
Rodrigues Solitaire (extinct)