An annoucement is sent to your contacts before your trip begins. When setting up your trip, you are given the option to notify contacts of the trip at a given time before your departure time, for example, 1 hour before you leave. Announcements allow you to notify interested parties that you'll be leaving, just so they know what's up.
An alert is sent to your contacts after you have failed to return from a trip. When setting up your trip, you are given the option to notifiy contacts after you have been overdue for a certain period of time. You can choose a different time period for each contact, so you can notify the cool-headed uncle Joe 1 hour after you're overdue, and hold off on notifying nervous aunt Peggy until you've been overdue for 5 hours.
When you create a trip, you have the option to setup both announcements and alerts, to be sent at a time of your choosing. For example, you may wish to notify your great aunt Betty 1 hour before you leave, and your grandmother Dotty 2 hours after you are overdue.
When the time arrives, an email is sent to Betty and Dotty, giving a link to view your trip on Odisius, where they'll be able to see your details, map, and personal information.