SharePoint is a collaboration platform and at the same time it is a development platform where developers can host different apps and take advantage of the communication and collaboration features provided by the SharePoint. SharePoint uses many different services and also integrates with other systems. It uses single or multiple servers on the backend to host and run SharePoint properly this part is usually taken care by the administrators.

So a developer develops apps for SharePoint, the apps could be client side and also could be of server side. If an app has its server side component usual an administrator tackle its deployment.

An administrator actually have the responsibility of managing SharePoint server and related services.