Animation Canceling is any special sequence of actions which speeds up actions occuring by "cutting off" the time taken to finish an animation. It doesn't have to be "click to move while an ability going off", that's just how it works in Riven's case. This is conceptually similar to an auto-attack reset, which is a skill that causes your auto-attack timer to reset, allowing you to auto-attack and then immediately auto-attack again by using the skill.
Orb Walking is technically not present in League of Legends. It was a very specific mechanic in Warcraft 3 / DOTA related to 'orb spells', which allowed you to attack an enemy without drawing minion aggro. Sometimes people use the term "Orb Walking" to mean using the time between auto-attacks to reposition, though it's a bit of a mis-use of the term.
Another similar term is "Kiting" which means to move as if there is a kite-string between you and an enemy. The term Kiting is most often used when a ranged character kites towards or away from a melee character, keeping them within range to attack while keeping them out of melee range. However, it has many other uses, including when you kite away from an npc that is aggroed to follow you, so he can't reach you.