Naturally, there are many spatial databases out there, and we can't show them all. For the purposes of this course, we'll use PostgreSQL and PostGIS.
PostgreSQL is a well-known database management system (DBMS), usually ranked in the top 5 DBMS in terms of popularity. We've chosen this particular DBMS because of its high level of compliance with the SQL standard, among other things.
PostgreSQL does not support spatial data management. That's where PostGIS has the floor - it is an extension that turns PostgreSQL into a spatial database. As we've already mentioned, PostgreSQL and PostGIS are not the only options for those interested in spatial databases. Other possibilities include Oracle with its Oracle Spatial feature, Microsoft SQL Server since version 2008, or MySQL since version 5.0.16.
Note that if you ever end up working with another spatial database, you'll have to devote some time to transitioning from PostGIS. You may have to check new spatial function names, as they may differ from those available in PostGIS. Still, the concept and the core features should remain the same regardless of the specific DBMS you use. That's because all of the popular DBMS implement Simple Features - a standard set that has been established by the Open Geospatial Consortium (OGC).