A casual friend or acquaintance is someone known through work or other mutual activities but who is not known well or personally. It is possible for casual friends or acquaintances to become real friends over time.
According to About.com, the way to test whether a particular person is a casual friend or acquaintance as opposed to being a real friend is to note whether the two people have done activities together outside work or whether they have bonded over a specific circumstance or activity. In addition, it is valuable to question how the other person probably considers the relationship. People can maintain casual friendships for years without them developing into deeper relationships.