There is an MSDN article on the same topic with same title. I am pretty much sure that you even tried to read that article and then posted the same title here with a plea to help you out.
http://msdn.microsoft.com/en-us/library/office/bb645998%28v=office.15%29.aspx[
^] Please read this article, get some concepts from it. There is a requirement for you to undergo and make sure that you're fulfilling them before you can understand the entire task. These include the knowledge of C#, Office API and the work you're going to do and what might the server require from you.
Once you've read it, and understood it. You will be able to write the applications yourself. The first thing that might confuse you is what the heck is all this code. That actually is the C# code, with just a little bit API language included, such as the Classes, members etc. Which might seem confusing to you right now.
Go through the API, their classes, members and their functions of what they do. Then you will understand the language yourself.
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