You need to activate it via steam before using. It will launch via steam (most likely) but you can put steam in offline mode. Obviously to play multiplayer, you will need steam online.
Imagine it is a CD key like it used to be. Instead of typing in the key, you simply let steam realise you have the game installed. It then notes that and that copy can't be used for anyone else.
edit: Empire: Total War will still most likely come with a CD key, and then you activate it as such:
Afterwhich it will appear in the Installed Games part of Steam.
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