Soundex is the name of a search algorithm that takes a word, such as a person's name, as input and produces a character string which identifies a set of words that are (roughly) phonetically alike. It is very handy for searching large databases when the user has incomplete data.
The algorithm is fairly straight forward for programmers to code, requiring no backtracking or multiple passes over the input word. In fact, soundex is so straight forward, it was done by hand by census takers in the U.S. around the turn of the 19th century (1880 to 1920).
The original Soundex algorithm (which was named simply "INDEX" at the time) was patented by Robert C. Russell in 1918. The method is based on the six phonetic classifications of human speech sounds (bilabial, labiodental, dental, alveolar, velar, and glottal), which in turn are based on where you put your lips and tongue to make the sounds.
A good article about SoundEx from Creativyst® Software is available on the web. It is entitled:
Understanding Classic SoundEx Algorithms