Though the name is now Messages, the features of iChat are more or less intact. The app still supports AIM, Yahoo, and Jabber protocols. (Since Google's chat system uses Jabber, Messages therefore supports Google Talk, too.) There's still a buddy list and still support for audio and video chats via those services. iChat Theater is still there, too—it's just called Theater now, and is accessible once you start a video chat from the Buddy List.
The major interface change in the upgrade from iChat to Messages is the new Messages window. This is a new, persistent window that collects all your currently-active conversations, regardless of which service they're on.
The left side of the Messages pane is a scrollable list showing every conversation, with the name and buddy icon of the person you talked to, a portion of the most recent message in the conversation, and the time or date the most recent message was sent or received. You can remove items from the left pane by moving your mouse over one and clicking the X icon that appears. Typing Command-W will also remove the currently selected item. (You can double-click on any of the items in the left pane to open them in a standalone window, if you prefer to have several, separate chat windows open at once.)
The right side of the pane is a chat window. There's nothing too dramatically different here, though when someone's typing you'll see the same thought-bubble icon that's on display in the iOS Messages app. The text-entry box at the bottom of the pane displays what service you're using for this particular chat, until you begin typing. At the top of the pane, there's a camera button that gives you quick access to video chat. FaceTime is the primary objective here: Messages will show you every phone number and email address for the person you're chatting with, in hopes that one of them will be valid for FaceTime use. If you just want to use AIM to chat with them, you have to scroll down to the Other submenu and then pick an account.
While Messages links to FaceTime, it's important to note that Messages doesn't actually do FaceTime. When you select a FaceTime address and initiate a FaceTime session, Messages opens the FaceTime app and initiates the connection. The two apps remain separate.