That's because, unlike iChat, it works with the iMessage platform that Apple . It still works with standard IM networks such as AIM and Jabber, and it can still send SMS texts to non-iOS phones.

But iMessage isn't just another messaging platform like those others. Rather, it ties your Mac and iOS devices into a single unified ecosystem. You can--in theory--begin a conversation on your Mac and then pick it up later on your phone. You can--again, in theory--see your entire chat history with a contact, regardless of the devices you used for those chats. And--for better or worse--when someone chats with you via iMessage, you'll be alerted on your Mac as well as on your phone and tablet.

New look and feel

Messages feels different from the moment you open it. Instead of iChat's single column buddy list and separate windows for chats, Messages's default interface has a two-column interface, with contacts on the left and a chatting pane on the right.

Instead of a long list of buddies, that left pane shows you a list of the people you've "conversed" with most recently--name plus the time or date and the last few lines of your previous exchange. You can sort that list chronologically or manually, dragging and dropping people into whatever order you want.