How customizable is the user-to-user access?

User permissions apply to an entire Web, not to documents themselves. However, you can have additional sub webs that can optionally have their own permissions. Each user can be given any of four default roles. Additional roles can be defined by the administrator.
Can each user have access to their own calendar?
Yes there are two ways to do this,
* by creating a calendar for each user, or
* by creating a calendar with a view for each user