the variables mouseX and mouseY always contain the values of where the cursor is. The variables pmouseX and pmouseY contain the previous value of the cursor position. For instance, to draw continuous lines, try this program:

Code:

 
void setup () {
  stroke(0);
};

//above sets the color of the line

void draw () {
  line(mouseX,mouseY,pmouseX,pmouseY);
};
 

There are infinite variations on this small program that really is purely user-controlled.

Hope this helps