PostgreSQL is a widely used open-source relational database system. It stores data in tables, uses SQL for queries and is known for its standards support, extensions and reliability. Many cloud database services are built on or designed to be compatible with it, including AlloyDB, which is why existing PostgreSQL tools and skills often carry over.
Compatibility is useful because it reduces lock-in and learning cost. Applications written for PostgreSQL can often be moved to a compatible managed service with limited changes, and the same drivers, clients and migration tools can usually be used.
Compatible does not mean identical. A managed service may support some PostgreSQL versions and extensions but not others, and it may change how certain features behave or restrict access to server-level settings. Performance characteristics, limits and pricing also differ from running PostgreSQL yourself.
Before moving an application, check which PostgreSQL version and extensions the service supports and which features are restricted. Test your queries, migrations and load against the managed service, and keep a plan for how you would export your data if you later chose to move elsewhere.