That's a pretty vague question, but you can pretty much do anything you can on any oter computer. The big seller on the Mac though is the way the applications are integtrated and things just 'work', like dragging pictures from iPhoto to iCal and still retaining the cropping and zoom tools you get in the former.
Being able to drag a picture from a web-browser onto the desktop, without right-click, save as, etc. Expose is great, F10 just isolates all windows for your current app, F9 shows all open windows and F11 moves everything to the side - I miss it at work when working with loads of apps.