Wikipedia definition of symbiosis: "Symbiosis (from Greek σύν "together" and βίωσις "living") is close and often long-term interaction between two or more different biological species."
Wikipedia definition of mutualism: "Mutualism is the way two organisms of different species exist in a relationship in which each individual benefits from the activity of the other."
In intro biology class, I (and many other people, I'm sure) was taught that mutualism is a type of symbiosis. However, now that I think more about it, it seems like some relationships that are mutualistic don't appear to fit the definition of symbiotic. Some relationships are clearly mutualistic and symbiotic (humans and gut flora, for example) but other relationships (like bees pollinating flowers) are mutualistic but don't appear to fit the definition of "close and often long-term interaction". The same of course goes for parasitic, ammensalistic, etc. relationships as well.