That vs which

Will have to think about this one for a bit but I think I get it …

That introduces essential clauses while which introduces nonessential clauses.

Examples:

“I do not trust editorials that claim racial differences in intelligence.” 
 We would not know which editorials were being discussed without the that clause.

“The editorial claiming racial differences in intelligence, which appeared in the Sunday newspaper, upset me.” 
The editorial is already identified. Therefore, which begins a nonessential clause. 
 
NOTE: Essential clauses do not have commas surrounding them while nonessential clauses do contain commas.

Source: http://www.grammarbook.com