A relative clause is an extra bit of information that tells us which (or what kind of) person or thing someone is talking about.  For example, if you are telling your mom about your friend, it may be helpful if you say "my friend Jana from work ...."  That way your mum can either connect other stories she's heard about this particular Jana or connect the story to a concrete person, especially if she has met her before.

When writing this part of a sentence, it is necessary to use one of the following words: which, that, or who.  "Who" is always used for a person.  "Which" is NEVER used for a person.  "That" can be used for either a person or thing.  Sometimes "who" is the only option for a person.  Don't worry about that for now!

Let's take a look at some examples of these now:

WHO

    "The children who live next door are very energetic." - we use "who" because we are referring to a person
    "The architect who designed our house is getting an award today." - we use "who" because we are referring to a person
    "We know many people who don't like to play sports." - we use "who" because we are referring to a person

WHICH

    "I don't know any company which makes modern furniture." - we use "which" for a thing
    "The application which that man handed us is on your desk." - we use "which" for a thing
    "The meat which we bought yesterday was very good!" - we use "which" for a thing

THAT

    "The woman that you didn't remember was the first one." - we use "that" for a person or thing

    "Everything that you did was horrible." - we use "that" for a person or thing
    "Our car that broke down has been fixed by my uncle." - we use "that" for a person or thing