What is Photon?
Photon by Exitgames is a collection of SDKs and services which provide user-friendly and effective multiplayer functionality useful for developing online games and other software.
Perfect for Unity!
Using the cloud package that was available for Unity, I was able to convert my non-multiplayer game into an online game in only a couple hours. I was even able to add a simple server browser which allows players to navigate current games being played, or create their own. The process really is quite easy and intuitive with Unity.
Photon Cloud Service
If you decide to use the Cloud service provided by Photon they will handle the server for you, and you will use a key to access their server through your Unity project. This puts less of a burden on you as a developer to maintain a server, but also has limitations and concerns including enforcing reliable networking communication to limit hacking. The Cloud service is free for a limited number of concurrent users (20), but a monthly fee can be paid in order to gain access to more concurrent users per server.
A Viable Solution
If it is ever necessary or desirable to develop a multiplayer game, using Photon should be strongly considered when working with Unity, as it largely supported and is also intuitive and easy to develop with. It is also useful for simply testing networking functionality since the 20 CCU hosted cloud service will not cost you a thing! And as always, there are plenty of resources and tutorials online to help you get started on your first multiplayer game.
Or just try my multiplayer platforming example!