After you launched your website, you may ask “OK. How can I measure number of people visited the site?”. That’s a quite important question in particular for tracking expenses on advertisement.
Also you may need to know
– where do website visitors live?
– how do they get to know about our site?
– what are their interests, age, sex, etc.
– what pages in the website are the most visited?
– etc.
In general there are two major approaches. The first one is the most simple: just install Google Analytics to your website, wait for a couple of days and start to study what could it tell you. It’s really simple, but will tell you only what Google want you to now. This is quite a big piece of information anyway, and for beginners it is much better than nothing.
Perhaps the major disadvantage of the first approach is that it could not be complete. For sample it could not always identify visitor’s age and sex. This information could only be available when visitor for sample have a GMail account and filled in this information in Google Plus profile. But what if the visitor have a Facebook account and use Yahoo mail? In such a situations you may try to use Facebook Insights for sample.
Another case is that you may be interested how any people shared a link to a page of site of yours over Twitter. For this purpose you’ll need to setup Twitter Cards.
For users who disable JavaScript (are there such a nerds still?) or use browser plugins that helps to avoid sending data to Google Analytics, you may need tools like AWStats.
And so on. I’d suggest to start from Google Analytics because it is the most simple one. And then make a list of questions you need to answer about your visitors. In case GA does not give you enough answers, try other tools.