At the ten-thousand foot view, .Net is essentially the same thing as Java, just made by Microsoft instead of Sun (and with everything that implies.)
Starting at about 9000 feet is where we point out that the two systems are utterly different from the ground up, are totally incompatible, but solve roughly the same problems in the same way philosophically, if not technically. Namely - they're both a combination of Virtual Machine Runtime running VM-specific bytecode and shipped with a massive library of pre-written code, libraries and widgets, as well as letting the VM handle things like memory management and garbage collection.
