JIRA stores its configuration and data in a database; if you are evaluating JIRA, then it's possible to use the embedded Hyper SQL Database (HSQLDB) written in Java. It's suitable for small applications and JIRA uses it only in its evaluation version. HSQLDB is not recommended for production usage. For that, JIRA recommends MySQL or PostgreSQL.
No matter what type of database is used, the database scheme, that is, the tables and the relationship between them is the same. If you want to take a look at the schema, then you can refer to JIRA_HOME/WEB-INF/classes/entitydefs/entitymodel.xml.
The contents of the file are as displayed in the following screenshot:
This is an XML file that contains the definition of all the tables in JIRA and its relationship with other tables.
Alternatively, you can also check the database schema on the Atlassian website at https://developer.atlassian.com/display/JIRADEV/Database+Schema.