Relative pronouns and adjectives refer to a noun or pronoun, previously mentioned in the conversation. They are shown in the table below.

One must remember:
quien is used for persons only.
el cual and que are used for persons and things.
Let's look at some examples of use:

Pronouns
Una chica a quien debes conocer.
A girl whom you should know.

El jefe es el que da las órdenes aquí.
The boss is the one who gives orders here.

Ese carro, el cual está muy sucio, es de Enrique.
That car, which is so dirty, is Enrique's.

Adjectives
Es la mujer cuyo hijo se ha muerto.
It is the woman whose son has died.

El café cuyo origen desconozco, me gusta.
The café, whose origin I don't know, I like.

La casa, cuya puerta está cerrada...
The house, whose door is closed...