NAMASTE -
1. To elders
2. Can be your neighbour
3. Your family member to whom you havent met or seen for long.
4. Guest
NAMASKAAR -
1. It's prefered to give it to your Teacher(Guru) which we call GURU in India in Hindi.
2. It's for 60+
Meaning wise both are same... In my place, we juniors' say NAMESTE and our parents, when they meet their friends they pass on sometimes NAMESTE sometimes NAMASKAAR to each other..