The purpose of open licensing is to allow people to reuse copyright material without having to contact the rights holder for permission. Whilst such licences are binding legal contracts, they are designed to be easy to understand and applied by non-legal people.
The most widely used of all open licences are the Creative Commons (CC) licences and rather than being all rights reserved (which is the default position of a copyright work) they are described as some rights reserved. They are modular in design, allowing for people to apply varying levels of restriction, from the most permissive (CC0 in which there are no conditions) to the most restrictive (CC BY ND NC users must attribute the source, cannot make derivative works and must not make the work available on commercial terms).
CC licences are used in a range of contexts from creative/artistic works to Open Access publications and major scientific datasets. Some examples include:
The licences can be used free of charge and can also be digitally coded into content and webpages so that search engines can filter out only Creative Commons licensed material. Use the Creative Commons search web page to search for material which can be used without needing to obtain further permission.
The UK Government has also released a Creative Commons compatible licence called the Open Government Licence (OGL), which allows reuse of Crown copyright material and public sector information. The licence has some additional restrictions regarding personal data and use of public sector organisation logos/insignia, but is largely equivalent to the Creative Commons Attribution (CC BY) licence.