Here we are then, the third and final law of Thermodynamics! Thus far you will have read all about the Zeroth,First and Second laws, and are now more than ready to explore further!

The third law of thermodynamics is really just about temperature, specifically: the lowest temperature it is possible to reach. This, in a rare moment of common sense, is called Absolute Zero! It sounds cool, but is in fact very cold! On the Kelvin scale of temperature, absolute zero is set to 0 K (deliberately, of course); this corresponds to −273.15°C or −459.67°F... so wear a hat!

Naturally, Absolute Zero is not merely a number thought up at random (though −459.67 does seem pretty random!), but is a direct effect of thermodynamics and, alas, entropy.

The most common statement of the third law is this:

As a system approaches absolute zero, all processes cease and the entropy of the system approaches a minimum value.

A convenient way to look at this, rather than get bogged down by the wording of a definition, is to think more along the lines of: “Absolute Zero is the point when all processes cease and the entropy of a system becomes zero”. This might seem like a re-wording of the above, but is in fact more a definition of Absolute Zero than of anything else, which is the point being made. At Absolute Zero a system has no energy, and no entropy: nothing happens at all, even at the atomic level.

Interestingly enough, it is theoretically impossible to actually cool something to absolute zero, though it is possible to get close (scientists have got down to 1×10−10 K, as quoted by Wikipedia, which is 0.0000000001 K).
Facts related to this!

1 - The background temperature of the Universe is 2.7 K (−271.15°C) and is called the Cosmic Microwave Background. This temperature, measured by telescopes such as Hubble and other specially designed ones (COBE, for example), provides an indication of how old the Universe is. By using the basic laws of thermodynamics (in fairly non-basic ways) and investigating how long the Universe would have had to cool to reach this temperature, the current fashionable theory (The Big Bang) puts the age of the Universe at 13.7 billion years!

2 - There is another thing called Absolute Hot (or the Planck temperature)! Where Absolute Zero is the coldest temperature anything can reach, Absolute Hot is the hottest: this is accepted as being 1×1032 K (a 1 followed by 32 zeroes before the decimal point!).