With Windows Vista you get a fully functioning Calendar, Contacts database and Windows Mail. It is like getting the Outlook program for free. You can access each program within the other as if they were one program. You could pay more money to get a few more features with an Email program like Outlook, but why would you when these are so feature rich?
Calendar, Contacts and Windows Mail is a suite of applications – these can and do link to one another. If you already use Outlook, their functions and features will be very familiar. If you are not familiar with Outlook, there's a short learning curve and a ton of information available to help you acclimate.