This is a definition for people that have absolutely no idea of what an operating system means. If you don 't understand what I just said, you need to read the following text. The people who do know what it all means, might take the following definition as incorrect, but that 's just so I can explain in simple language what I mean.
“Trinity Rescue Kit" or TRK is a collection of programs that can start a computer in an alternate way if it failed to start normally.
TRK is a so called "live distribution" of Linux. Linux is in fact the "brain" and "the senses" that drives your computer, all the programs are the limbs that allow you to control it. All this put together make up a so called operating system. Because of confusion in terminology and because of the so many flavors, we talk about Linux being the operating system and what makes it complete with programs is called the distribution. And as for distributions, a lot of flavors exist (hundreds, maybe even thousands). One of these flavors is Trinity Rescue Kit. TRK is a "live" distribution because it can boot from removable media, perform hardware detection on-the-fly and automate as many configuration tasks as possible.
TRK operates completely from RAM (= compare this with the short term memory of your brain) and read-only media. This means also it doesn ‘t touch nor change anything stored on your computer until you tell it to do so. Another implication this will have is that by starting your computer with Trinity Rescue Kit you have the guarantee that no viruses that might be on your local computer can become active. In the case of your computer running Microsoft Windows in normal circumstances, it just is impossibe by design that a windows virus can run on TRK because TRK is Linux.
You have the possibility of using 5 different antivirus scanners with TRK (current version of this writing: 3.4). One of them, the free open source Clamav is integrated in the distribution, the other 4, F-prot, Bitdefender Vexira and Avast get downloaded from the Internet when launched. For Avast you need a free license key handy, for which you need to register on their site.
More on what TRK is can be read on the frontpage