Google Trends is one of the most understood tools in the online marketing world. Most users seem to think that they just have to enter two terms and then see which of the both terms is more popular in terms of search volume. In fact, this is not the case. Google Trends displays the search interest as a measure of search volume relative to all searches on a particular day. As a result, a curve that goes down may not mean that there were less searches, there could have been even more from one day to another. But if the overall search volume has risen faster, then the search interest for that particular term has decreased while it’s volume has increased.
In addition, data is indexed. This means that data is always interpreted from the highest point in the set, resulting in search interest curves crossing while search volume curves would not.
One result of Google Trends was Google Flu Trends, a product that identified a correlation between searches for flu-related terms and actual flu penetration. However, as it turned out later, flu predictions proved to be inaccurate.