collinear represents two lines lying on the same straight line.

First and foremost, please understand the difference between a ray, a line and a line segment.

|------------------------------------> = Ray

<------------------------------------> = Straight line

|--------------------------------------| = Line segement

<---------------||--------------------> = Two rays whose measurement is 180 degress but no vertex is said to exist, just the meeting point of the two starting points of the rays. If the two rays form an angle other than 180 degrees or 360 degress, there would be a vertex.

PS:- I have tried to explain from whatever I learnt in my early school days and that was very long back. Please correct me if I am wrong, Mods/Members.