The difference between the 2 depends on a lot of things such as location and tradition. In certain places, they are used interchangeably. However the easiest way to separate the two is that colleges typically offer courses up to undergraduate level while universities offer undergraduate and graduate courses.
Some colleges offer graduate courses but this is not very common. In most cases they are colleges that have upgraded to Universities but cannot change the name due to certain reasons such as wanting to maintain the traditional name of the school.
Another fact worth noting is that a University can have a college(s)