MacBook Air will bring Retina Display and new internal components, namely the CPU, the graphics card and probably some more RAM.
MacBook Pro is the second updated product and Apple is expected to release two versions of the 15-inch model, with a design change, Retina Display and new internal components.
Nothing is certain regarding the 13 and 17-inch MacBook Pros, but Cupertino is rumored to have changes in plan for these models as well.
Regarding the iMac and the Air, the device will have new internal components, but it is unknown if this product will feature Retina Display and apparently no changes will be made in terms of design.
Nothing is certain regarding the Mac mini and Mac Pro, except in the case of the former as a new Mac mini is rumored to be in the cards for WWDC.
Finally the WWDC will also see the launch of several new accessories for its products including refreshed keyboards, mice, trackpads, AirPorts, or anything else.