Capistrano is a utility and framework for executing commands in parallel on multiple remote machines, via SSH. It uses a simple DSL (borrowed in part from Rake, http://rake.rubyforge.org/) that allows you to define _tasks_, which may be applied to machines in certain roles. It also supports tunneling connections via some gateway machine to allow operations to be performed behind VPN's and firewalls.
Capistrano was originally designed to simplify and automate deployment of web applications to distributed environments, and originally came bundled with a set of tasks designed for deploying Rails applications. The deployment tasks are now (as of Capistrano 2.0) distributed as a separate package: capistrano-deploy.
Capistrano is "opinionated software", which means it has very firm ideas about how things ought to be done, and tries to force those ideas on you. Some of the assumptions behind these opinions are:
By default, the script will look for a file called one of +capfile+ or +Capfile+. The +someaction+ text indicates which task to execute. You can do "cap -h" to see all the available options and "cap -T" to see all the available tasks.