DuckDB is an in-process (now I guess less so with Quack) OLAP database for analytical workloads. PostgreSQL or MariaDB are OLTP row-oriented databases that are great for application/transaction-focused workloads but are less great when you want to query across a giant amount of data.

DuckDB is much like SQLite, but built for OLAP workloads: it's in-process, with a single file format on a disk, and (unlike SQLite) the data is stored in columns for better OLAP performance.

Like SQLite, concurrency options are limited compared to client-server databases like Postgres or MariaDB. DuckDB 2.0 will be adding a client-server mode with the Quack protocol which can allow for greater concurrency.

But you can also use DuckDB as a multitool to connect to and query all kinds of other data sources from one connection. Being able to pull in data from Postgres, CSVs or parquets on a file system or S3, and JSON returned by a web API, and then query across all of it in one place, can be quite handy for ad-hoc data analysis and exploration.