There are still classes, however, they work a bit differently. Every class has it's own Powers List that it has to choose from. There are 4 different types of powers. At will, Encounter, Daily, and Utility. At will is exactly that. You can do it at any time. Encounter powers can be used once an encounter (theory is that you need to have a short rest 5-10 mins before you can accomplish that again), and teh daily power is once per day. Utility powers are generally not attack powers, but things such as extrodinary moves. For example, tumbling through combat spaces, or a spectacular standing long jump, etc....
As for the wizard, he still has the spells, but they are not nearly as "catastrophic" as they used to be. Magic Missile has become the at will spell (not the only one) and changed a little bit. You must now roll to hit, but the damage starts a bit higher too. (2d4). Really, there are too many change to discuss just one, but as I am finding out quickly, so far I like most of the changes, but we have encountered a few things that may/maynot be mistakes. I plan on contacting Wiz Co about them as soon as I can put it all down in to some semblance of order.