That and Which
“That” is restrictive; it is used to draw the reader’s attention to a specific aspect of the information you are giving. Because the information is essential, the clause is not set off with a comma. “Which” is non-restrictive, it is used to add information. This information is not essential to the meaning of the sentence, and therefore set off with a comma.
For example:
I will wear the dress that is in the laundry.
I have more than one dress, but I want to wear that one.