What do I need to bring with me on my trip to Europe?

I'm going to Austria/Germany/France/Czech Rep and who knows where else for almost 4 weeks. What all do I need to bring? How much clothing? Also it's wintertime so I need to take that in consideration. I don't want to lug around a lot of crap in Europe. I plan on maybe buying some stuff over there. But any tips advice about what to pack would be awesome, I'm leaving from Seattle on the 16th.