Relative Velocity

[ritwik06]

The problem I am facing is conceptual. I come across many questions in which they say that the velocity of a person or boat is say 3 m/s with respect to water. Then they give the stream velocity.

whats the use of the stream velocity when I know the velocity of boat or swimmer with respect to water??? Moreover in certain books, I have seen that the subtract the stream velocity from the horizontal component of the swimmer's or boat's velocity(wrt to water) to calculate the drift. I am totally confused.
Please explain to me 3 terms in more detail:
1)Velocity of river with respect to ground
2)Velocity of swimmer with respect to river/water
3)Velocity of swimmer with respect to ground

[heth]

I typed relative velocity river simulation into Google and it gave me this page:

http://qbx6.ltu.edu/s_schneider/physlets/main/boatriver1D3.shtml

It may help conceptually.