iTunes 11 has undergone many different visual tweaks and changes, not all of which may be welcome. One of the big ones is how the Search field works. By default, when you type in your search term, a drop down menu appears under the Search field, listing the songs, albums, or apps that include those search terms. This can be handy for searching the whole of iTunes at once, but you might want to use the older style of search, using the sidebar to filter the search results within the different media types.
Here’s how to revert iTunes 11 to the older style.