is it just an installer or something?

It’s a compressed installer to be precise. Usually crackers only put up the patch files, which is pretty useless for anybody wanting to pirate a game. Fitgirl repackages (and compresses) the game files together with the crack patches into a cracked installer, hence “repack”.

And if you imported the game into steam for example can you be “banned” or VAT (idr the term)

Nope, though it will look funny in your Steam Status there is nothing to worry about. After all for all Valve knows you could be using a renamed version of a legitimate App, for non-steam entries in steam only the .exe name is known, nothing else.

Just a word of warning: be careful when torrenting the games, while Valve will not ban you for using them game publishers are known to sue when catching you torrenting them (usually the upload part in the torrent is what gets people but the downloading is not 100% safe either). You should definitely look into how to safely torrent stuff before committing to it in order to avoid any nasty cease-and-desist letters flying in your door.

The compression was already explained - I would add that if you’re not deep into scene groups, popular repackers are in general a safe choice, i.e. no malware included. Of course it’s never 100% safe, but in my opinion safer than picking from a list of torrents what could be the scene release.

I never had issues with fitgirl and Dodi, maybe other users can drop some more names that have proven to be reliable over a long time.

I’ll add that if you want to archive games forever, storing repacks is a good idea because of their extreme compression. From what I’ve observed, Fitgirl trends towards heavier compression while DODI trends towards faster install times.

KaOsKrew is another respected repacking group that you can trust.