I love to swim but I am not sure if it is good on the body, do you know?
I love to swim too. Swimming is actually a low impact activity and compared to running is better on the body.
Do you work a lot of muscles? I know my body is normally tired when I am done.
Yes.  It's a great workout.  I suppose it's different for people as swimming relies on the natural buoyancy of the body.
Does it have any long term effects? Like I know running is bad on the knees long term.
No long-term effects that I know for sure, but since the human body is less dense than water, water supports your weight while swimming.  No negative impact to the body at all.