Scripting languages are programming languages that are interpreted as opposed to programming languages that are compiled (converted into binary executable before being able to run). Scripts are interpreted instruction by instruction each time they are run. They offer authors a means to extend HTML documents in highly active and interactive ways.
A client-side script executes on the client's machine when the document loads and when various well defined user actions take place, such as link activation when a button is pressed.
Server-side scripting is as the name implies a server-based technology, which can be used to dynamically generate HTML pages.