Google indexes an amazing 200 million web pages, using 3,000 computers. Google has become very popular with those who prefer speed and simplicity, and so far - no ads!
Google Scholar lets you search "specifically for scholarly literature, including peer-reviewed papers, theses, books, preprints, abstracts and technical reports from all broad areas of research."
Google Local helps you focus your search on a specific geographic location. For now, Google Local only searches for locations in the United States and Canada.
Yahoo approaches the Web through over a dozen major categories, and remains the main access for users who prefer a guided approach to web searching. Yahoo is the #1 visited place on the entire Internet.