This is quite easy, really: "that" defines, "which" gives extra information (often in a clause enclosed by commas):
This is the house that Jack built; but this house, which John built, is falling down.
The Guardian, which I read every day, is the paper that I admire above all others.
I am very proud of the sunflowers that I grew from seed (some of the sunflowers);
I am very proud of the sunflowers, which I grew from seed (all the sunflowers).
Note that in such examples the sentence remains grammatical without "that" (the house Jack built, the paper I admire, the sunflowers I grew), but not without "which"