A database (DB) is a set of information stored in a structured way so that it can be found, modified and protected efficiently, managed by software called a DBMS (database management system). An online store's customers, their orders and the product catalog all live in a database.
It is queried with queries. Relational databases store data in linked tables and are queried in SQL: PostgreSQL, MySQL, SQLite, Oracle, SQL Server. They guarantee transaction consistency, which is essential in finance or logistics.
NoSQL databases drop the table model for other forms: documents (MongoDB), key-value (Redis), wide columns (Cassandra), graphs (Neo4j). They serve specific needs such as an ultra-fast cache or highly distributed volumes. On top of these come analytical data warehouses (Snowflake, BigQuery) and the vector databases used by AI.
Why it matters when hiring
Being able to write an SQL query does not make someone a database expert. For a back-end or data role, a good signal is the ability to explain an index, an execution plan or a modeling choice that prevented a performance issue. Do not confuse a DBA (who administers and tunes databases), a Data Engineer (who builds data flows) and a back-end developer (who uses them).