Tau (uppercase/lowercase Τ τ), is the letter of the Greek alphabet, used to represent the "t" sound in Ancient and Modern Greek. In the system of Greek numerals, it has a value of 300. Letters that came from it include the Roman T and Cyrillic Т.
Greek alphabet  

Αα  Alpha  Νν  Nu 
Ββ  Beta  Ξξ  Xi 
Γγ  Gamma  Οο  Omicron 
Δδ  Delta  Ππ  Pi 
Εε  Epsilon  Ρρ  Rho 
Ζζ  Zeta  Σσς  Sigma 
Ηη  Eta  Ττ  Tau 
Θθ  Theta  Υυ  Upsilon 
Ιι  Iota  Φφ  Phi 
Κκ  Kappa  Χχ  Chi 
Λλ  Lambda  Ψψ  Psi 
Μμ  Mu  Ωω  Omega 
Other letters  
Ϝϝ  Digamma  Ⱶⱶ  Heta 
Ϻϻ  San  Ϙϙ  Koppa 
Ϡϡ  Sampi  Ϛϛ  Stigma 
Ϸϸ  Sho  

Other Uses
Some people want to use Tau in place of Pi, the special number that comes from circles. Tau would be equal to two times pi, or about 6.283.
These people want to use Tau because they think it would be easier for everyone to understand. There are many reasons in math why Tau could be useful or helpful. One of the simplest is that it would make learning about radians easier.
A radian is a way of measuring the angle of a circle. It says that the number of radians in an angle is equal to the length of the outside of the circle, or the 'Arc length', that is covered by an angle, divided by the Radius, or distance from the point in the center of a circle to anywhere on the outside. There is a picture down on the bottom right that shows this.
If the distance covered around the outside of a circle is equal to the radius, than one radian is being covered. Around the whole outside of a circle, there are about 6.283 radians  or, Tau radians (Tau is just a number, like 0, or 7, or 100).
The advantage of this is that if you turn one quarter of a circle, you will have turned Tau / 4 radians. If you turn one half of a circle, you have covered Tau / 2 radians. This might make more sense. You can see a drawing of this in the bottom right of the page.
Not many people use Tau yet, but more and more are.