It is the line of symmetry of a parabola and divides a parabola into two equal halves that are reflections of each other about the line of symmetry.
It intersects a parabola at its vertex.
It is a vertical line with the equation of x = -b/2a.
Any point that is co planar with one another and is also equidistant from a given point is a circle. We need it to be co planar, otherwise it would be 3-dimensional and create a sphere. And if they we not all equidistant, they would not make an even circle (it'd be an oval or another shape).