Semi-structured data is a type of data that does not conform to a strict schema but still contains some organizational properties. It often includes tags or markers to separate different elements, making it easier to analyze than unstructured data. Common examples of semi-structured data include XML files, JSON documents, and HTML web pages. This format allows for flexibility in data representation, accommodating varying data types and structures. Semi-structured data is widely used in applications like NoSQL databases and data interchange between systems, as it can efficiently store and transmit complex information without rigid constraints.