Why can't fresh water and salt water mix? What happens when they collide with each other? What are their differences?
Strictly speaking, they do mix and if you take some salt water and fresh water and try it yourself as pointed out in the comments I think it would be pretty obvious that they do (eventually).
However, the initial "separation" that happens is because of the fact that salt water is denser than fresh water - as such it will sink through the water (while slowly diffusing). If left alone long enough, diffusion will cause the salt ions in the salt solution to mix with the fresh water, resulting in the homogeneous mixture.
I googled up for you a little bit. This large-scale phenomenon is quite interesting and has been discussed from different points of view.
To quote from the physics answer:
There are two mechanisms for mixing at a liquid-liquid interface, firstly diffusion and secondly physical agitation.
Diffusion is negligably slow in liquids, it takes days for solutes to travel a few centimetres, so the mixing is dominated by physical agitation e.g. wave action, convention currents, wind mixing etc.