What's the difference in Qué and Quién? How to use what and where in Spanish

Quick definitions:

1. Qué: a direct translation of 'what'. Qué is used when referring to objects.
2. Quién: is used to ask the question 'who'. This is used when referring to people.

Plurals

Qué does not change regardless of the number of items being referred to.

Quién specifically is used when asking a question and the expected response is a single person. If the expected response is more than one person, then 'quiénes' is used.  

Accent marks - when to use them?

The accents are used when a question is being asked. If who is used to add information about a person, then que or quien are used without the accent mark.

My mother who is German, works as a nurse.

Mi madre que es alemana, trabaja como enfermera.

Wait, why did you use que for who?

Generally speaking, when adding information about a person, que is used. If there is a comma between a named person and who, a comma can be used. However, que tends to be used most often.