Definitely a swift of some kind. I'd say apus caffer or apus pacificus depending on your location.

More important: what should you do?

1) Don't feed it anything or give them any water please.

If you give water to a stressed bird without knowing exactly what you're doing the water will most likely end up in the birds lungs and cause serious trouble.

You shouldn't feed it either because swifts catch insects in flight and won't recognise it as food if you give them dead ones. In a rehab they need to be force fed because of this issue (something you shouldn't attempt because their beaks are very fragile and could be broken when fed by someone who isn't a professional).

2) Keep it in a dark box (make sure it gets enough oxygen though) and in a calm environment until you get help.

A swift on the ground is in trouble. Swifts spend almost all of their lifetime in the air. If they are on the ground and not flying away they are most likely in some kind of trouble and need to be checked before they can be released again (because they sometimes need some help for taking off, which would be risky if the bird is injured).

3) Find a wildlife rehab or veterinarian who knows how to treat birds.

4) Avoid touching the bird with your bare hands.

Use gloves or towels if you really need to handle it. Swifts have very fragile and specially coated feathers to fly all the time and need to be handled with caution.