(The former homepage http://alpha.olm.net/ is no longer available, and all Alpha proponents should update links on their web pages.)
Alpha is a very powerful, multi-purpose text editor.
Alpha is multi-modal, which means it switches between different modes depending on which type of document you are editing. Everybody who has used Emacs knows about this concept. The idea with different modes is to change Alpha's behavior depending on which kind of text you're editing. For example if you're programming in C++ or editing a LaTeX document you want different features.
Alpha is very customizable. Much of Alpha's functionality is implemented in 'packages' or 'extensions'. The "Config --> Preferences --> Features" and "Config --> Mode Prefs --> Features" menu items let you see which items are installed, and lets you turn them on and off.
Alpha uses Dr. Ousterhout's Tool Command Language (Tcl) as an extension language. Many of the functions bound to keystrokes, as well as many of the functions in the menus, are written in Tcl. All the "Packages" or "extensions" just mentioned are all written in Tcl. Anybody who wants can write new packages for Alpha!
Of course, you don't have to learn Tcl to use Alpha. You can have lots of use of Alpha without knowing anything about Tcl. But if you want to become a real power user, you should consider trying it out. Then the possibilities of customizing Alpha are unlimited.
From "Learning Alpha" by Profiles.DonavanHall
Alpha is an extensible, cross-platform text editor. This description falls short of encapsulating what Alpha is. Because Alpha is extensible, it can be made to do much more than simply edit text. Built into the Alpha application is a scripting engine that allows a user to script custom functions and to add user-defined menus. The scripting language used to extend Alpha is called Tool Command Language or Tcl for short. Tcl was developed by Dr. John Ousterhout, who now heads Scriptics Corporation, the company that continues to support and develop Tcl. (We will return to Tcl later.)
While the power (and beauty) of Alpha is its extensible nature, you don't need to be programmer (or scripter) to benefit from Alpha's fine editing modes and features. Each of Alpha's editing modes offers a few standard features (such as automatic structuring, keyword highlighting, and typing shortcuts) that assist in the preparation of specialized types of text files or documents. Currently, Alpha provides over thirty different modes.