Google Trends is a Google Labs graduate that allows you to see what others have been searching for with Google. Google Trends graphs how often a term is used over time and where geographically most people are searching for a given term. You can also compare more than one term to view relative popularity. It's a less sophisticated version of other Google analytic tools, such as Google Insights for Search.
Google Trends also includes the lists of Hot Topics and Hot Searches.
Hot Searches measure searches that are the fastest rising at the moment. This isn't an absolute measure of popularity, as the most popular searches tend to stay the same according to Google. Trends are relative increases in popularity, such as searches for the latest gadget announcement or music video release.
Hot Topics measures trending terms in news and social media feeds like Twitter. This isn't a measure of searches, but it is a measure of what people are talking about on the Internet.
Google Trends is on the Web at www.google.com/trends