Server hosting game

Servers are just computers. If it runs Windows, it can run the game .exe file. Be sure your server has enough graphics power to handle running the game if you have 3D rendering enabled on the server.

Linux servers cannot be made with Unity (unless you run them with WINE, but that's probably not a great idea). And if, for some strange reason you have an OSX server, you can run the OSX-built binary (.app) that Unity creates as the server. Windows and OSX clients will be able to play on the same server.

Unity uses its own networking, but if you want to, you can create your own server in whatever language you want, and then you can (instead of using Unity's Network Views), code your own networking into your game that works with the server you've created. You're then free to use whatever software you want as the server for your game (Perl, Python, C++, C#, Java (not Javascript), or whatever).

A dedicated server typically utilizes a Client-Server architecture, where you have a (dedicated) server running somewhere that is NOT also a game. Other clients then connect to the server and play the game.

A Local server that still uses the Client-Serverarchitecture is when you run an instance of the game that doubles as the server for the game. A great example of this technology is the Source engine, which has the capability to run either a local server or a dedicated server.

The other way you can run a server is using a Peer-to-Peer architecture. In this instance, the game contains both "server" and "client" code, and two players can connect to one another and play against each other using exactly the same game code, without any real server.