I've always been taught that metaphors and similes both draw a parallel between two disparate ideas/thoughts/objects, but that a simile is a more explicit comparison using the word "like" or "is", whereas a metaphor's connection is more implicit. For example, "His injured ankle burned like a hot stove" is a simile, where as, "His injured ankle was a hot stove of pain" is a metaphor.
However, I recently heard a colleague note that similes are nothing more than a special kind of metaphor. In other words, all similes are metaphors. This seems to contradict what I was always taught (and what I've read online, for what that's worth). Can anyone straighten me (or my colleague) out?