Difference between StandardController and StandardSetController?

ApexPages.StandardController encapsulates just a single Sobject (e.g. Account, Opportunity).

ApexPages.StandardSetController contains a list of records (one or more), and has additional functions to facilitate pagination (moving between pages)
and updating a number of records at once.

For example in test class when we initialize

ApexPages.StandardController

Account objAccount = [SELECT Name FROM Account LIMIT 1];
ApexPages.StandardController standctrl = new ApexPages.StandardController(objAccount );


ApexPages.StandardSetController

List<account> accountList = [SELECT Name FROM Account LIMIT 20];
ApexPages.StandardSetController ssc = new ApexPages.StandardSetController(accountList);