These people usually have some sort of stats tracker. For example if your hosting provider offers you CPanel as a control panel, it usually has Awstats and Webalizer to help you track your visitors. Some people use other products as well such as Google Analytics, etc.
These products usually provide you with information on how many visitors you've had each day and month, where they are from in the world based on their ISP, what site referred them to you, what keywords they used in specific search engines to find your site, and so on.