Oracle Database (commonly referred to as Oracle RDBMS or simply as Oracle) is a multi-model database.

It is the world's most popular database for running online transaction processing (OLTP), data warehousing (DW) and mixed (OLTP & DW) database workloads. The latest generation, Oracle Database 12c Release 2 (commonly referred to as 12.2), is available on-prem, on-Cloud, or in a hybrid-Cloud environment. 12.2 may also be deployed on Oracle Engineered Systems (e.g. Exadata) on-prem, on Oracle (public) Cloud or (private) Cloud at Customer (e.g. Exadata Cloud at Customer). At Openworld 2017 in San Francisco, Executive Chairman of the Board and CTO, Larry Ellison announced the next database generation, Oracle Autonomous Database.

Oracle Database Cloud Services
Customers can choose from a wide range of database services on Oracle Cloud that are available as 'pay as you go' hourly and monthly rates, and also have the option to re-use existing on-prem licenses on Oracle Cloud via a Bring Your Own License (BYOL) program. 

Oracle Database On-Premise Licensing
Prospective purchasers can obtain licenses for on-prem deployment based either on the number of processors in their target machines or on the number of potential seats ("named users"). The term "per processor" for Oracle Database Enterprise Edition is defined with respect to physical cores and a processor core multiplier (common processors = 0.5*cores). e.g. An 8-processor, 32-core server using Intel Xeon 56XX CPUs would require 16 processor licenses. The low entry cost, Oracle Database Standard Edition 2 (SE2), includes Real Application Clusters and may be deployed on servers or clusters with a maximum of 2 sockets total.

Oracle Database Express Edition (XE) offers a free version of the Oracle RDBMS, but one limited to 11 GB of user data and to 1 GB of memory used by the database (SGA+PGA). XE uses no more than one CPU and lacks an internal JVM. XE runs on 32-bit and 64-bit Windows and 64-bit Linux, but not on AIX, Solaris, HP-UX and the other operating systems available for other editions. Support is via a free Oracle Discussion Forum only.