1f56bc31_ox_vs_App___SharePoint_Dragons__Farm
[0] Server-side code [Sandbox] Runs under a strict CAS policy and is limited in what it can do. [Apps] No SharePoint server-code. When apps are hosted in an isolated SharePoint site, no server-code whatsoever is allowed. [Farm] 
Can run full trust code or run under fine grained custom CAS policies.
SharePoint 2013: What to do? Farm solution vs Sandbox vs App | SharePoint Dragons
Farm