Theorem: If two lines intersect, they intersect in exactly one point.
Postulate 1: One line contains at least two points.
Postulate 2: Through any two points, there is exactly one line.
Postulate 3: Through any three noncollinear points, there is exactly one plane.

If you don't know the difference between theorem and postulate, a theorem tells the truth and can be proven, while postulate is only assumed to be true without any proofs.