Let’s face it, we often have a definition problem.
It’s critical to distinguish “open licenses” from “public licenses” when discussing IP licensing, especially online — mostly because Creative Commons is so popular and as a result has muddied the waters a bit.
Open has so many meanings to people (same of course as with “free software” or free cultural works) that it is critical to define from a legal perspective what is meant when one says “open”. The Open Knowledge Definition does this, and states that “open” means users have the right to use, reuse, and redistribute the content with very few restrictions — only attribution and share-alike restrictions are ok, and commercial use must specifically be allowed.
The Open Definition means that only two out of the main six CC licenses are open content licenses — CC-BY and CC-BY-SA. The other four involve the two non-open license elements the No Derivatives (ND) restriction (thus prohibiting reuse) or have Non Commercial (NC) restrictions. The other four are “public licenses”, in other words they are licenses provided for use by the general public.
Of course CC’s public domain tools, such as CC0, all meet the Open Definition as well because they have no restrictions on use, reuse, and redistribution.
The Open Data Commons legal tools, including the PDDL, the ODbL and the ODC Attribution License, all comply with the Open Definition, and so are all open public licenses.
I haven’t done a full survey, but the majority of open licenses (in terms of popularity) probably also fit the definition of public licenses, as open license authors tend to draft licenses for public consumption (and these tend to be the most used ones, naturally) . Many open licenses aren’t public licenses though — mainly those drafted for specific use by a specific licensor, such as a government or business. So the UK government’s new Open Government License isn’t a public license because it’s not meant to be used without alteration by other governments, but provided it meets the definition of the Open Definition, would be an Open License.
A simple Venn Diagram might be: