TYPO3 uses a database (typically MySQL) and the file system to store data. The file system keeps configuration files, some cache files, images, and uploaded files. The database stores pages, content elements, and lots of system data (such as TypoScript templates, logs, and so on).

All (to be precise most, but for our purpose, all) tables in TYPO3 roughly follow the same structure. They have a set of predefined (reserved) fields. TYPO3 will not work properly if one or more of the required fields is missing. Examples of predefined fields are uid (unique identifier of the record), pid (id of the page where this record is located), crdate (record creation time), tstamp (last update time), cruser_id (uid of the Backend user, who created this record). A table may also contain other reserved fields. If it does, TYPO3 will automatically provide additional functionality for the table. The best examples of such fields are deleted (indicates whether a record is deleted), starttime (indicates when a record becomes visible in the FE), endtime (indicates when a record stops being visible), and hidden (indicates whether a record is hidden). There are other fields, which will be discussed later. All these fields are managed by the system, and extensions usually do not change them.

TYPO3 comes with several default tables. These main tables are:

If an extension provides a new table, it must ensure that the table name has a certain format. The table name must start with tx_, followed first by the extension key without underscores and next by an underscore, and the table name. For example, an extension with the extension key my_ext can have the following valid table names:

The following table names are not valid:

We will discuss tables in more detail when we generate extensions later in this book. At the moment, it is important to remember two things: