Up to five terms can be compared. And you can also view queries that contain either or two terms, using a vertical bar “|”. More advanced queries can be done as well – see the FAQs for details. Google also puts markers next to major news events that are about that search query, helping to explain surges. Data can be sorted by time, language, geographic location, etc.
In testing it I’m finding it works well for very highly searched terms, but terms that are very rarely searched show no data at all. Seeing trends on even obscure terms would be useful. Even so, Steve Rubel says Google Trends is “a must-bookmark for every PR person and marketer worldwide.”