When you are doing something you want to know if it working or not. The same with website or application you want to know how many people using it. There are some tools to provide data in some reasonable statistics view. I’m using Google Analytics tool.
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Its popular and well documented tool. It is ready for almost all cases you can imagine within measuring user activity on your website. Can provide nice overview on all devices or deliver statistics on regular basis

Example of user sessions in Google Analytics