online learning blackboard; whats it called?

[answer]

I remember seeing this software used for teaching people; where a student is at home, instructor is at his office; both infront of a computer. The instructor can draw and write into the software and the student can too, and both see an updated image, as the instructor explains to the student visually. E.g. instructor draws a line, student see's line being drawn in "real time".

Whats it called; and what are some varients?

Thank you