Roleplaying, to me, implies meaningful interactions with a gameworld that are unique to my character based on who they are. For instance, let's say I make a character who is strong, dumb, and evil...
Pretending: When interacting with the gameworld, there's nothing to show that my character is physically stronger than any other character, but I still imagine them as a strong brute. When they interact with NPCs, they have the same dialogue options as anyone else, despite their low intelligence. And if they go around killing people in a certain area, the NPCs act like nothing happened when I return to the same area at a later date.
Roleplaying: The game allows my character to move a boulder out of the way which another, weaker character cannot. My character cannot learn languages a more intelligent character could master. My character has simpler dialogue options than an intelligent character. If my character kills someone in a village, they earn a sinister reputation among the NPCs.
This really sums it up for me.
Roleplaying is imagining something about your character that is supportable in-game.
Pretending is imagining something about your character that is not supportable in-game.
Neither is right or wrong, their just different ways people use to enjoy games. I myself find pretending in games to be dull, I greatly prefer it when the game world reacts to my rp choices.