A profile in Outlook contains your email addresses, contacts, calendars, and preferences. You can have multiple addresses within one profile, or you can use your profiles to separate your different inboxes.
For Outlook 2007 and earlier, an Outlook profile can only have one Exchange address in it. But it can have multiple IMAP or POP addresses in it. Starting with Outlook 2010, you can have multiple exchange addresses in a single profile.
Think of Outlook profiles like different user logins on a single computer. Each user has a desktop, different applications, and their own files and folders. Similarly, each Outlook profile has its own setup, and data.
You can't switch Outlook profiles while Outlook is running.
Outlook will start with the default profile. You can set Outlook to prompt you for a profile when it starts.