asi file that you drop into the same folder, though some may have a few extra files. That's pretty much it! Most script mods you download will have a single.
On Steam, it's 'Steam\steamapps\common\Grand Theft Auto V'.
Take dinput8.dll and ScriptHookV.dll (and the trainer, if you want it) and put them in your GTA 5 game directory, wherever GTA5.exe is located. It allows for all sorts of cheats in single-player, like changing your skin, changing the time of day, teleportation, invincibility, and more. In the zip file, there's a folder called 'bin' with three files in it. Just be sure to use the download link inside the table on the bottom-right: there are advertisements on that page that also say they are downloads. Here is the link Blade provides to download ScriptHook.
