Piracy: ꜱᴀɪʟ ᴛʜᴇ ʜɪɢʜ ꜱᴇᴀꜱ

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.

It's a cracked version of the game that has been heavily compressed to reduce download size. This is why the installer will eat a lot of CPU resources during installation. While you probably won't be banned, I would generally avoid playing these games through steam as a precaution. Usually multiplayer doesn't work normally anyways.

Repacks are stripped down & compressed versions of other releases from scene or P2P. Stuff like VC/DX redistributables will be removed, and extra languages are usually separate so you can choose not to download them. They are usually a few gigs smaller than the original releases, but come at the cost of much longer installation time & CPU usage to decompress everything. They also sometimes come with updates and DLC pre-applied.

In my opinion if you have decently fast internet there generally isn't really a reason to prefer them over original scene/P2P releases. You also implicitly have to trust both the original release and whoever made the repack - FitGirl in this case. But even if you trust them there are impersonators and reuploaders that will bundle legitimate repacks with infected junk.

Edit: There is also no risk associated with adding pirated games to your Steam library as a non-Steam game.