Help about Oracle licensing
Oracle licensing is a complex topic. You can find information on the Oracle pricing page. 

You can find there links to a paper called "Software Investment Guide" that has a lot of licensing samples.
Also there can be found some papers about special licensing topics and a price list.

There is also a FAQ concerning Oracle licensing (not an Oracle site) that may be interesting.

DISCLAIMER: All information is offered in good faith and in the hope that it may be of use, but is not guaranteed to be correct, up to date or suitable for any particular purpose.

Oracle has several different types of "editions" of its database prodcut: 
Enterprise Editions, Standard Edition, Standarad Edition One. Enterprise Edition has additional options must be licensed if enabled.

Oracle has two types of licenses: so called "Named User Plus" licenses and "Processor" licenses. You can either license the machine where Oracle is used. Then you have to use Processor Licenses.  Or you can license the users/devices that are able use the database. Then you have to use "Named User Plus" Licenses. 

If you use Enterprise Edition the number of Processor Licenses needed is the number of cores multiplied by a factor that depends of the processor type. I you use Standard Edition (Standard Edition One) the number of needed Processor Licenses equals the number of processors in the machine. You cannot use Standard Edtition (Standard Edition One) on all machines: machines that have or can have more than 4 processors cannot be licensed with Standard Edition . For Standard Edition One the limit is 2 processors. 

If you use Processor Licenses you always have to license whole machines except for some special situations (look at the "partitioning" document in the "Speciality Topics" section of the Oracle pricing page) where it is possible only to license a part of a machine. If you have a cluster , failover, backup or test environment you havee  to check how to  license this environment additional machines.

If you use "Named User Plus" licenses than bear in mind that a user is not a database user but a person or device that uses the database. So for example if you have an application that is accessible by all of your employees and this application stores its data in an oracle database then all of your amployees need "Named User Plus" license. Even if the application uses only one database user.

There is also a minimum number of "Named User Plus" licenses that is needed depending on the number of Processors/Cores that are used for Oracle databases

A machine with 2 intel processors and standard edition  needs 2 processor licenses if one uses processor licensing.

If enterprise edition is used  it depends on the number of cores. The core factor is 0.5 for Intel processors. For  2 Hexacore processors  2*6*0.5=6 "processor licenses" are needed. The same number of licenses is needed for every licensable option, too.