BootCaT uses the Bing search engine to find web pages relevant to your domain. Unfortunately, in order to perform this kind of automated search, you need to register for an account that limits the number of queries that can be submitted (this limitation is enforced by Bing to prevent abuses).
At the time of writing, you can get a free account on the Windows Azure Marketplace that lets you submit 5.000 queries a month (every tuple you create with BootCaT becomes a query, so if you generate 50 tuples, when you click on “Collect URLs” in BootCaT you will use 50 of your queries). If you need more than 5.000 queries a month, you could try signing up for a paid account (see below) although this has not been tested yet: if you do decide to buy a paid account, you do it at your own risk.
An Account Key is a sort of password: it's a very long string of letters and numbers. Unlike a password, you don't get to choose what your Account Key is, it's assigned to you and you can't change it.
To get your free Windows Azure Marketplace Account Key:
Remember that you don't need to create a new Account Key every time you install BootCaT, you can use the same key on multiple computers.
NB: it's advisable to use copy/paste to insert the Account Key in the relevant box inside BootCaT, you must use the keyboard shortcut CTRL-V to paste the key in the box. @Mac users: please note that the keyboard shortcut is CTRL-V (and not CMD-V) on Macs too.
BootCaT is a free software program developed by people at the universities of Bologna, Trento and Zagreb and is in no way affiliated to Bing, a commercial service owned by Microsoft. The main reason for using Bing to retrieve URLs is the fact that it offers a limited number of free queries a month: we don't get a cut if you buy a paid account from the Windows Azure Marketplace, nor are we responsible for anything related to Bing.
VERSION 0.60 STOPPED WORKING ON AUGUST 1, 2012, IF YOU WANT TO USE BOOTCAT YOU NEED TO UPGRADE TO VERSION 0.61
See this page to determine which version of BootCaT you are using