At the date of writing, Odoo's latest stable is version 8, marked on GitHub as branch 8.0. This is the version we will work with throughout the book.

It's important to note that Odoo databases are incompatible between Odoo major versions. This means that if you run Odoo 8 server against an Odoo/OpenERP 7 database, it won't work. Non-trivial migration work is needed before a database can be used with a later version of the product.

The same is true for modules: as a general rule a module developed for an Odoo major version will not work with other versions. When downloading a community module from the Web, make sure it targets the Odoo version you are using.

On the other hand, major releases (7.0, 8.0) are expected to receive frequent updates, but these should be mostly fixes. They are assured to be "API stable", meaning that model data structures and view element identifiers will remain stable. This is important because it means there will be no risk of custom modules breaking due to incompatible changes on the upstream core modules.

And be warned that the version in the master branch will result in the next major stable version, but until then it's not "API stable" and you should not use it to build custom modules. Doing so is like moving on quicksand: you can't be sure when some changes will be introduced that will make you custom module break.