CSOM vs JSOM vs SSOM vs REST

CSOM: 

   Client-side object model. C# (or Visual Basic) only, 
JSOM: 

• JavaScript object model. JavaScript only.
• SP.ClientContext.get_current() 
• new SP.ClientContext('url...') 
• works cross-SPSite in 2013.

SSOM: 
    Server-side object model. C# (or Visual Basic) only,

REST:

 REST web technologies and standard Open Data Protocol (OData) syntax.

Either JS (JavaScript) or C#