Weird connection between circles and lines

When I draw a circle and then draw a straight line and I lay the line over the circle through a diameter - they become somehow permanently glued together! I can imagine this might be useful in some cases, but once they’re joined, I cannot find any keystroke that will allow me to move one without moving the other or even deleting one without deleting the other.

How do I separate these things and what is this “arrangement” called?