Thank you for the explanation & the link. It was very informative.There are two types of focus sensors in DSLRs. Vertical Line Sensors and Cross-Type Sensors.
Vertical Line Sensors are one dimensional contrast focus points. They detect and focus on contrast along a vertical line and are generally considered to have a lower accuracy except for when detecting contrast along a vertical line.
Cross-Type Sensors are two dimensional contrast focus points. They detect and focus on contrast along both a vertical and horizontal line. Thus providing higher accuracy in focus.
There is a good article on Autofocus and how it works here: