There is a theory circling the internet about how all Pixar movies are all apart of one big universe...And it all make total sense and in a way makes all of pixar's movies better. This theory was made by a guy called Jon Negroni
http://jonnegroni.com/2013/07/11/the-pixar-theory/ - This is Jon Negroni's typed out theory, it is the most in-depth as well.
And here is a picture timeline giving you a visual.