In American English, "that" is typically used for restrictive clauses, which provide essential information about a noun. Often, the sentence will no longer be complete if you remove the word "that" and the clause that follows it. 

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In the first sentence, you can see "that" also used as a demonstrative pronoun, but we are interested in the first "that," which functions as a relative pronoun.

And I bought  a voltage converter that will convert that down to twelve volts.

Caption 39, Hobbies and Projects: The Electric Cargo Bike Project

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It's not too early to pack on the pounds that will get them through the next winter.

Captions 17-18, America's National Parks: Olympic

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Actually, there is one thing that Kate's always had a little trouble with.

Caption 43, ALF: Working my way back to you

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As a relative pronoun, the word "which" is reserved for non-restrictive clauses. These offer additional, non-essential details and are usually set off by commas. Note that in the examples below, you could stop the sentence before "which" and it would still be complete. 

And now for the final touch, which is the bread.

Caption 62, Abiventures: Sandwich

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One of the largest tributaries into the canyon is the Little Colorado, which joins the big river inside the park.

Captions 17-18, America's National Parks: Grand Canyon

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The mule deer bucks are re-growing their antlers, which they shed each winter.

Captions 31-32, America's National Parks: Yosemite

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Further Learning
Still wondering what a demonstrative pronoun is? No worries! Next month, we will look at "this," "that," "these," and "those." In the meantime, practice using "that" and "which," and look for more examples on Yabla English.