Reply to The difference between DJing Monitors & Studio Monitors?

Hey, I am a beginner Dj have done more than couple of gigs now.
I own a Cdjs and a mixer and I plan to buy Djying Monitors for my home studio.
My questions..

1. What is a difference between a Djying monitor and a Studio monitor.
2. Can Djying monitor be used as a normal Speaker for loud hearing purposes?
3. When do I need an amplifier?
4. Please also suggest good brands for studio monitors?- Decent brand(Medium Budget)

Your help would be appreciated