During the course, we will study and analyze the main and fundamental aspects the information retrieval systems (IRSs) are based on. In particular, the course will focus on web-based IRSs. Different kinds of IRSs will be studied, providing an as much as possible complete and up-to-dated picture of the IRS services now available and used by all of us. These IRSs will be analyzed and compared, showing their pros and cons, differences and similarities. Several examples of actual and commonly used IRSs will be provided, showing how previously studied theoretical principles are applied and how current systems are evolving including new and peculiar features.