This post is directly from Google Help. I doubt I could say this better than the source, so giving it to you straight.
Download the tutorial PDF here.
Based on search queries, Google Trends lets you see what people want to know about, all across the world over time. Its data goes back to 2004, so you can also see how trends have peaked and dropped, and it covers Web Search, Image Search, News Search, Google Shopping and YouTube Search.
Google Trends gives marketers a serious advantage. You can monitor interest in your brand, compare it with interest in your competitors and track shifts in your industry; view current trends to see if your marketing’s in line, take advantage of Trending Searches to expand your reach and follow rising trends to get a step ahead with your planning.
You could look for consumer interest across the year:
Or maybe you want to check out whether there’s more interest in Adele than in Taylor Swift? Simply type in your search terms and start filtering to see trends over time and geographic location. (For the record, Taylor shakes Adele off in India but is way behind her in France).
Or perhaps you’re marketing trainers in Italy. By comparing interest in various cities, you can see that Turin would be a better focus than Milan.
For both real-time marketing and long-term planning, check out Google Trends now.