That vs. which

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That
/ðət/ (Conj.)
Which
/wɪʧ/ (conj.)

That is a conjunction introducing a restrictive subordinate clause.

“I fed the cat that sat on the mat.” [implied: ... and I let the other one, which sat in the armchair, go hungry].

To be contrasted with which, being a conjunction that introduces a non-restrictive subordinate clause:

“The cat, which sat on the mat, had halitosis.” [Its halitosis was not contingent on its location mat-wise; indeed there could be an entirely distinct minty-breathed moggy sitting on the same mat, for all we know.]