A hidden shadow world that tracks Internet users' behaviors lies behind everyday websites. Web tracking analyzes online behaviors based on collected data and provides content that may interest the user. Web tracking collects vast amounts of data for various purposes, from sensitive personal information to trivial information such as settings and preferred information types, including the user's IP address, device, and search history. Although web tracking is largely a legitimate technology, there is a steady increase in illegal user tracking, data leakage, and illegal sale of data. Therefore, the need for techniques capable of detecting and preventing web trackers is becoming more important. This paper introduces a general description of web tracking technology, related research, and a website measurement tool that can identify web-based tracking. It also introduces techniques for preventing web tracking and discusses future research directions.