Defining vs Non-defining Relative Clause

Defining relative clauses provide essential information about a person or thing, while non-defining relative clauses provide extra information that isn't necessary:
Defining relative clauses
Provide essential information that's necessary to understand the sentence. For example, "They're the people who want to buy our house".
Non-defining relative clauses
Provide extra information that's not necessary to understand the sentence. For example, "Clare, who I work with, is doing the London marathon this year". 
Here are some differences between defining and non-defining relative clauses:
Pronouns
In defining relative clauses, the pronouns "who," "whom," and "which" are often replaced by "that" in spoken English. In non-defining relative clauses, you cannot replace other pronouns with "that".
Punctuation
Non-defining relative clauses are always separated from the rest of the sentence by commas, unlike defining relative clauses, which have no punctuation.
Relative pronoun
In non-defining relative clauses, you cannot leave out the relative pronoun, even when it is the object of the verb in the relative clause.
Here are some examples of defining and non-defining relative clauses:
Defining relative clause
"I'm going to wear the skirt that I bought in London". The defining relative clause tells us which skirt the speaker is wearing. The sentence would have a different meaning without the defining relative clause. 
Non-defining relative clause
"He gave me the letter, which was in a blue envelope". The non-defining relative clause gives more information about the letter, but it doesn't tell us which letter it is. 
Here are some more examples of defining and non-defining relative clauses:
Defining relative clause: "They have two sons who are doctors". The defining clause indicates that there are more than two sons in the family. 
Non-defining relative clause: "They have two sons, who are doctors". The non-defining clause indicates that there are only two sons in the family. 
Defining relative clause: "She gave me the cupcake that was red velvet flavored". 
Non-defining relative clause: "She gave me a cupcake, which was red velvet flavoured".