I have created a file that consists of a triangle and a circle. The circle looks like this:
<circle
style="fill:#000c00;fill-opacity:1"
id="path4142"
cx="642.11932"
cy="459.29367"
r="35.673294" />

My next step is to use this image in another product. I need to make this circle responding to mouse clicks. for that I need to know how to interpret the coordinates above. Can someone please explain what these coordinates represent?

Thanks

Current stabile release of inkscape doesn't have a circle object.

The cx and cy coordinates locate the origo of the circle.
If it was made with a pre-release, that means a position right and above the (0;0) point, because inkscape's coordinate system is flipped.
The r value shows the length of the radius of the circle.

Probably all of the values represent pixel dimensions.


Another product?
May the information in the svg specs help you with that.

What is a 0;0 point? Left lower corner of the picture?

The lower left corner of the page border.