Burp is a platform for performing security testing of websites, including (list taken from the Burp website):
- An intercepting Proxy, which lets you inspect and modify traffic between your browser and the target application.
- An application-aware Spider, for crawling content and functionality.
- An advanced web application Scanner, for automating the detection of numerous types of vulnerability.
- An Intruder tool, for performing powerful customized attacks to find and exploit unusual vulnerabilities.
- A Repeater tool, for manipulating and resending individual requests.
- A Sequencer tool, for testing the randomness of session tokens.
- The ability to save your work and resume working later.
- Extensibility, allowing you to easily write your own plugins, to perform complex and highly customized tasks within Burp.
More information - Burp Suite on Wikipedia