So one of the good ways is always to start from the basic things !
Direct digital synthesis (DDS) is a method of producing an analog waveform— Sine wave, Square wave or Triangle are the most used and required— by generating a time-varying signal in digital form and then performing a digital-to-analog conversion.
Square, triangular, and sinusoidal waves from a DDS
There are many possibilities for frequency generation , ranging from phase-locked-loop (PLL)-based techniques for very high-frequency synthesis, to dynamic programming of digital-to-analog converter (DAC) outputs to generate arbitrary waveforms at lower frequencies.
A good example can be a signal generator, an essential tool in the arsenal of any electronics geek !!
It is also widely used in Radio frequencies, and modulation .
So i came across this Article on Direct Digital Synthesis. http://lionel.cordesses.free.fr/gpages/DDS1.pdf
Also check these webcasts from Analog Devices... http://www.analog.com/en/content/WC_FUN_PLL/webcast.html
Last but not least, the good ol'Wikipedia