Why You Need Databases
You need databases because you need to persist data. You can't rely on data sitting on the client or the application server—what if the server reboots, or the user closes their laptop or browser? Databases are the main way we persist data because their job is to store it and retrieve it efficiently. Every application has to store and retrieve data, and we usually do that with databases. The two main types you'll work with are SQL and NoSQL. Neither is inherently better; each has its use cases. Pick the best one based on the problem.
SQL (Relational)
Traditional relational databases—PostgreSQL, MySQL, MariaDB—store data in tables with rows and columns. Think of it like Excel: structured, tabular. They're usually ACID compliant, meaning transactions are atomic, consistent, isolated, and durable.