1. Data
2. Relationships
3. Constraints
4. Schema.

1. data
2. programs.

Data are binary computer representations of stored logical entities. They are distinct pieces of information, usually formatted in a special way.

The term data is often used to distinguish binary (machine-readable) information from textual (human-readable) information. For example, some applications make a distinction between data files (files that contain binary data) and text files (files that contain ASCII data).

A program is a collection of instruction for manipulating data. Data can exist in a variety of forms - as numbers or text on pieces of paper, as the plural of datum - a single piece of information. In practice, however, people use data as both the singular and plural form of the word.

Data dictionary is a file that defines the basic organization of a database. A data dictionary contains a list of all files in the database, the number of records in each file, and the names and types of each field. Most database management systems keep the data dictionary hidden from users to prevent them from accidentally destroying its contents.

Data dictionaries do not contain any actual data from the database, only book-keeping information for managing it. Without a data dictionary, however a database management system cannot access data from the database.
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