The indices are accumulated in the index files. The data is accumulated in the data files. Indices give fast access to data items. For example, a book database may be managed in the order of Accession number, yet may be indexed on Book titles and Author name.
Data Dictionary: A Data Dictionary kept information about the structure of the database. It is used a lot. Therefore a good data dictionary should have a good design and efficient execution. It is seen that when a program becomes somewhat big in size, keeping track of all the available names that are used and the purpose for which they were used becomes more and more hard. After a significant time if the similar or another programmer has to change the program, it becomes extremely hard.
The problem becomes even more complex when the number of data types that an organisation has in its database enhancing. The data of an organisation is a valuable corporate resource and thus some kind of inventory and catalog of it must be maintained so as to assist in both the management of the resource and utilisation.
It is for this purpose that a data dictionary or directory / dictionary are emerging as a main tool. A dictionary gives definitions of things. A directory notifies you where to find them. A data directory / dictionary have information (or data) about the data.
A comprehensive data dictionary would give the definition of data items, how they adjust into the data structure and how they tell to other entities in the database. In DBMS, the data dictionary kept the information concerning the conceptual, external and internal levels of the databases. It would join the source of each data field value, that is from where the authenticate value is obtained. The audit trail regarding the updates and frequency of its use including user identification with the time of each update is also recorded in Data dictionary.