A calendar license key stops working on a particular date. After that, you cannot use MainBoss until you get new keys. Calendar keys were created to let potential customers try out MainBoss for a limited period of time.
Version keys are issued to actual MainBoss customers who have paid to license the software. A version key means you can use a particular version of MainBoss "forever"—the key will never expire for that version.
More precisely, a version key lets you use any version of MainBoss Advanced released before a particular date. For example, if you have a version key good to January 1, 2012, it will let you use any version of MainBoss released before that date. The key never expires, so you will always be able to use the version of MainBoss that you currently have. However, the key will not be good for versions of MainBoss that were officially released after the key's expiry date. If you wish to use a newer version of MainBoss, you need to obtain newer keys.
In practice, releases of MainBoss are usually backdated to allow flexibility of schedules. For example, MainBoss Advanced 3.4 accepts version keys dated January 1, 2012 or later, but the software was actually released several months afterward.
MainBoss Advanced also accepts Demonstration keys. These keys never expire, but they place restrictions on your MainBoss use. In particular, you are only allowed to create a limited number of work orders, requests, and purchase orders.