They basically act as a "browser console". From within your browser, they allow you to type out a command and have it performed in real time. For example, you can type 'd', tab complete it to "define", and then type any word you'd like to define. The definition will be shown as you type. You can even highlight words in the text of the page and then start ubiquity/taskfox and type "define". Ubiquity/Taskfox will understand that you mean to define the highlighted word and perform accordingly. Of course, a feature to define words is only one of many. With Ubiquity you can search Google or Wikipedia, translate between languages, send the current page as a email, post to your twitter, get maps and driving directions, and perform tons of useful tasks, all from typing a simple pseudo-english phrase of what you want.
EDIT: You might find this useful: http://vimeo.com/4062903