Easter Eggs. That's what they call them, because they're hidden and if you don't know what to look for, you might miss them. They're actually kinda cool!
Like this one. The garbageman in Toy Story 3 is that rotten kid from next door in Toy Story. Sid, the one who takes apart all the toys. He's the same guy and you can tell because he's wearing the same t-shirt. Hey, at least he has a job, right?
And in Monster's Inc, you can see the monster practicing his camouflage in front of the same wallpaper in Andy's room in Toy Story.
Kinda cool, right?
Check out this video with 84 Easter eggs in Pixar movies!