Oracle database software is available in following different editions. The basic concept of having so many editions is, based on requirement people choose their editions.
i.e Oracle Database enterprise for large enterprises and Oracle Database Lite database to run on mobile devices.
Oracle Enterprise Edition:
Enterprise Edition is the full (top of the range) version or the Oracle Database Server. Options like RAC, Partitioning, Spatial, etc. can be purchased separately to enhance the functionality of the database. This edition typically runs on servers with 4 or more processors and it has no limit on memory (RAM). Clustering feature is available in this edition using Oracle RAC (Real Application Cluster) software. Moreover, it has more features than Standard Edition. Oracle Corporation licenses this product on the basis of
users or of processors.
Oracle Standard Edition:
Standard Edition is designed for smaller businesses and enterprises. It offers a subset of the features/ functionality implemented in Enterprise Edition. Database options like Data Guard, Partitioning, Spatial, etc. is not available with Standard Edition (from 10g one can use RAC with Standard Edition). Standard Edition can only be licensed on servers with a maximum capacity of four processors and it has no limit on memory (RAM).
Oracle Standard Edition One:
Standard Edition One is a low cost, entry-level version of the Oracle Standard Edition database server. Standard Edition One can only be licensed on small servers with a maximum capacity of two processors. Moreover it has no memory limitation. Can’t use RAC with Standard Edition one.
Oracle Personal Edition:
Personal Oracle is a single user version of the database server. It is mostly the same as Enterprise Edition, but doesn't support advanced options like RAC, Streams, XMLDB, etc.
It is licensed for single-user. This edition is for single processor workstations.
Oracle Database Lite:
Oracle Light is a database engine that can be used on mobile platforms like cell phones and PDA's. The database itself is located on the mobile device and is having the ability to
synchronize with the main Oracle server.
Oracle Express Edition (XE):
Express Edition (XE) is a free, downloadable version of the Oracle database server. Oracle XE can only be used on single processor machines. It can only manage up to 4 GB of data and 1 GB of memory. ISVs can embed XE in 3rd party products and redistribute it freely.
After making a decision regarding which edition you need, the next thing is to find out how you want to have the licensing of the software. You can buy the licenses based on the number of users using the Oracle Server or based on the number of processors you will be having in the server machine.
To view these edition differences further more you can go through below link: