A StandardController is a built-in controller available for standard and custom objects to facilitate the easy creation of Visualforce pages.
Standard controllers provide access to object fields, standard actions (Save, Cancel etc.) and related lists without the need for custom apex code of any kind.
Basic example of standard controller use:
<!-- this will allow the user to edit the name of the account referred to by ID in the URL: -->
<apex:page StandardController="Account">
<apex:inputField value="{!Account.Name}"/>
<apex:commandButton action="{!Save}" value="Save!"/>
</apex:page>
The documentation for the StandardController apex class can be found here.
