User accounts are at the heart of any Unix-based operating system, including Mac OS X. The entire system relies on this concept, so having a basic understanding of what accounts are and why you need them will help you better comprehend and use Mac OS X.
Since Unix-based operating systems rely on the concepts of permissions and ownership, each file, folder, and application must belong to a specific user. For this to be the case, users must be declared and identified; hence the idea of creating unique accounts for each user, much as every customer of a bank has a private account that no one else can access.
When you set up or receive an account on Mac OS X, you become a user of that computer, and you are assigned a