The Neuter Relative Pronouns "Lo Cual" and "Lo Que"

Quick Answer

The neuter relative pronouns lo cualand lo queare used to refer to an abstract situation or concept that is not specifically masculine or feminine. While they are very similar in meaning, lo cual can only be used in specific contexts.

examples
El niño gritó en voz alta, lo cual me molestó mucho. The child screamed in a loud voice, which upset me very much.
Lo que dijo no tiene sentido. What he said doesn’t make sense.

Lo Cual

Lo cual can only be used to refer to something that has already been mentioned in the same sentence.

examples
Él siempre se jacta, lo cual me molesta mucho. He always brags, which really annoys me.
Hay un libro nuevo por Isabel Allende, lo cual me emociona mucho. There is a new book by Isabel Allende, which I'm really excited about.

Lo Que

Lo que can be used any time lo cual is used, and it can also be used to refer to something that has not been brought up before in the same sentence.
Lo que can also be used at the beginning of a sentence.
It's better to use lo que instead of lo cual when using non-specific pronouns such as eso, esto, todo, etc.

examples
No puedo contarte lo que escuché en la escuela hoy. I can’t tell you what I heard at school today.
Lo que dijo el presidente es importante. What the president said is important.