Cucumber is a tool that supports Behaviour-Driven Development (BDD) - a software development process that aims to enhance software quality and reduce maintenance costs.
Avocado is an open source interaction design toolbox built by IDEO. It enables designers to make quick interactive prototypes without writing a line of code. Built on top of Facebook's Origami framework, Avocado provides ready-to-use patches that can be easily combined to create fully-customized prototypes. Official announcement: http://labs.ideo.com/2014/05/27/avocado/
It is behaviour-driven development, Python style. It uses tests written in a natural language style, backed up by Python code.
It is python’s xUnit style framework. It works much the same as the other styles of xUnit, and if you’re familiar with unit testing in other languages, this framework (or derived versions), may be the most comfortable for you.