Rastic, you need to understand the concept of a relative pronoun. Start by reading the DL Tips and Notes for this lesson.
In English, the one word "who" can be used both as an interrogative pronoun and as a relative pronoun:
1. Who is the man standing next to my wife? [interrogative]
2. The man who is standing next to my wife is my brother. [relative]
In Dutch, the who in 1 is a different word from the who in 2 (wie vs. die).