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 ...