Discrete Fourier transform
Wikipedia would be a better place to get a full answer, but in short: Discrete Fourier Transform is defined over a finite set of $N$ samples $x_n$ by the formula 
$$
X_k=\sum\limits_{n=0}^{N-1} x_n e^\frac{-i2\pi kn}{N}  
$$
The resulting DFT coefficients contains information about the frequency content of the original signal.