Commonly, it is a wide-format image (whereas avatars are often square or nearly square in proportion). It often appears at the top of the profile; the user’s name and/or username, along with their avatar, may be drawn over the cover photo.
On touch interfaces, the cover photo is often vertically cropped until the user pulls the content down.
Some implementations do a similar thing on classical computers, as well; e.g., Facebook makes users scroll up to view full cover photos (except on your own account, where it is all visible initially. Those sneaky designers know we are vain.)
This largely needs sourced, but: I understand cover photos were adopted in this order: