There are many types of information, and many ways of representing the content of information. However, for visual information, there are many more ways to characterise the picture. We may use such features as colour, texture and shape. Exactly which aspects of an image are important depends on the application, and therefore no single method of describing the content of a picture will suffice for all situations. This means that we need great flexibility in a database used to store and retrieve such information.
Traditional databases enable exact-match querying of information.
The aim of information retrieval is to find documents about a topic. This gives rise to the need for inexact, or best-match queries. Each candidate is assessed to determine the extent to which it satisfies the query. Some number of these results which best match the query can be returned. Relevance can be very subjective, and so it is a good idea to give the user a chance to indicate what is relevant and what is not.