Emacs 25.1 is the current version as of this writing. A version number with two components (e.g., ‘24.5’) indicates a released version; three components indicate a development version (e.g., ‘26.0.50’ is what will eventually become ‘26.1’).
Emacs is under active development, hosted at Savannah. Follow the instructions given there to clone the project repository.
Because Emacs undergoes many changes before a release, the version number of a development version is not especially meaningful. It is better to refer to the date on which the sources were retrieved from the development repository. The development version is usually quite robust for every-day use, but if stability is more important to you than the latest features, you may want to stick to the releases.
The following sections list some of the major new features in the last few Emacs releases. For full details of the changes in any version of Emacs, type C-h C-n (M-x view-emacs-news). As of Emacs 22, you can give this command a prefix argument to read about which features were new in older versions.