Google Analytics is able to track enormous amounts of data for you:
- where did your visitors come from
- what pages visitors did and did not visit
- how long your visitors stayed
- where they were located (city, not street location)
- what type of device they were using, and what sized screen they have
Using the tool, you can then parse out further information and compare the data over time, location, etc.
Essentially, you can track the How and Who of your visitors: who came, how they got there and the technology they were using at the time.
What the data misses -- and cannot track explicitly -- is Why and What.
- why did your visitor visit your site and
- what were they looking for when they were there (unless they use search while visiting.)
Remember: Google Analytics cannot track your visitor's reasoning behind their decisions -- it can only track their actions: their path through your site. It is important to remember this as you interpret the data you collect!
The closest way to determine answers to why your visitor came and what they were looking for is to look at the search data: what search terms they used to find your site or once they arrived. This does, however, leave out the visitors who did not use search in association with your site.