What is the difference between Client Side Controller and Server Side Controller in Salesforce Lightning?

The framework supports client-side (JavaScript) and server-side (Apex) controllers. An event is always wired to a client-side controller action, which can in turn call an Apex controller action. For example, a client-side controller might handle an event and call an Apex controller action to persist a record.
Server-side actions need to make a round trip, from the client to the server and back again, so they usually complete more slowly than client-side actions.