Google Trends gives you a snapshot of the interest around a particular keyword over time. The tool shows you the degree of interest and not absolute numbers around how many people searched for it.
It is not
It is not a sophisticated SEO or PPC tool such as my current favourite SEMRush. For example, you will not be able to tell which keywords your competitors are appearing for on the first page of Google.
Quite simply because it gives you a very quick introductory picture of your chosen market/audience as well as being dead easy to use.
You can also see whether news headlines had any effect on the search interest and data can sliced up by countries, regions, cities and language.