The word "calendar" is an overloaded term. Bring it up in a meeting, and you are likely to find someone thinking of scheduling meetings, another thinking of a public events listing, and yet another of the academic calendar. When discussing calendaring, it is useful to break the topic down into subtopics.
Calendaring can be roughly divided into four areas:
1. Public events calendaring
2. Personal events management (PIM)
3. Group calendaring (e.g. scheduling meetings)
4. Resource scheduling (e.g. room and AV scheduling)
Most calendaring systems handle only a subset of these four areas, and when searching for solutions, it is helpful to identify what kind of calendaring you are looking for.