What is Maltego?  Well according to their website, it is:

Maltego is an open source intelligence and forensics application. It will offer you timous mining and gathering of information as well as the representation of this information in a easy to understand format.

What does that mean?

It means that Maltego is a commercial($) tool, that when provided a person’s name, email address, website, etc…, can quickly search for and identify related information from numerous sources on the Internet.

Maltego is particularly useful in scoping for a penetration test or social engineering engagement.  Using Maltego one can enumerate employee names, email addresses, phone numbers, postions, as well as alternate websites, dns entries, and so on.

Maltego comes with a number of built-in transforms.  A transform is a module which Maltego uses to perform a particular information search.  A varied collection of user created transforms can also be found and integrated with Maltego.