The words used to describe these kinds of clauses vary a lot: I like Descriptive and Defining.
American English has a tendency to use “that” to introduce defining clauses, and “which” or “who” (always preceded by a comma) for descriptive clauses – I think this is a good idea – but it is not a “rule”. (In British English, this tendency is not so marked.)
As general guidance,
When written, descriptive clauses are always off-set by commas.
When written, defining clauses are never off-set by commas.
Descriptive clauses can always be omitted because they give unimportant or unnecessary information – information that is an aside or a casual remark.
Defining clauses cannot be omitted. Defining clauses tell you which precise item is being referred to.
1 Wife to her husband: “The cat that I saw yesterday is in the garden again.” – “The cat that I saw yesterday” is a single noun phrase acting as the subject, and “that I saw yesterday” defines the cat – it tells you which cat it was – it was the cat that she had seen yesterday and no other cat.
If the wife had omitted that I saw yesterday, and said “The cat is in the garden again”, her husband would have said “Which cat are you talking about? You have never mentioned a cat to me!”
2 Wife to her husband: “The cat , which I saw yesterday , is in the garden again.” = “The cat (and, by the way, I also happened to see it yesterday,) is in the garden again.” Also, the sentence assumes that her husband knows which cat it is.
If the wife had omitted , which I saw yesterday, and said “The cat is in the garden again”, it means that she is aware that her husband already knows which cat she was talking about.
"I saw a car which was white."
This would be a defining clause - there is no comma - and it might be a reply to "There are no white cars in this area."
"She went into the apartment, which had blue walls."
This would be a descriptive clause - there is a comma (and "which" has been used) - It means "She went into the apartment(that had been mentioned early) and, by the way, the apartment just happened to have blue walls, but they could have been any colour and it would not have made any difference."