Technology

ELI5: a database is the "memory" of a program.

Every piece of data that any software uses almost certainly comes from and goes to multiple databases.

Once the data is stored, you can execute "queries" to have powerful access to update many records at a time, read particular records based on their relationship to other records, and so much more.

Your bank balances, your purchase history, your emails, every part of your digital life is almost certainly spread across a constellation of databases.

Bonus Fediverse content:

Lemmy itself uses the Postgres database extensively. Posts, users, comments, votes and more are all individually stored in the database.

Mastodon also uses Postgres. If a post goes up on Lemmy, and a Mastodon server is federated with it, the Lemmy server will send out a HTTP request to the Mastodon server containing the contents of the post. The Mastodon server will use this information to write its own record of the post in its own database.

Regarding your question about VMs: You can run a database inside a VM, or give the VM access to an outside database via queries, or both! You might run SQLlite (a small and excellent embedded database) on the VM to track its local state, while also running queries against a large postgres database to synchronize with other services in the cluster.