Calendars spins off the task manager part of iCal to its own app called Reminders. Tasks in iCal always felt kind of tacked on and I never used it very much. Reminders brings the task list more front and center.
Address Book is now called Contacts. It is pretty much the same app, just with a new name.
Mail is still called Mail, but the notes part of Mail has been spun off to its own app called...wait for it....Notes. To me, this is a great change because I use Notes all the time on the iPhone but rarely use them on the Mac. It never made any sense to me to hide notes inside of Mail.
iChat has had both a rename and a huge makeover. It is now called Messages.
All of these apps sync between all of your Macs and iOS devices using iCloud just as they always have. The difference is that now all of the apps have the same name on all of Apple's products which makes it easier to move between OS X and iOS.
In future tips I will discuss the features of the new apps Reminders, Notes and Messages as well as discuss the changes and improvements in Calendar and Contacts.