When properly used, these programs allow you to parse complex languages with ease. This is a great boon when you want to read a configuration file, or want to write a compiler for any language you (or anyone else) might have invented.
With a little help, which this document will hopefully provide, you will find that you will never write a parser again by hand - Lex & YACC are the tools to do this.
Although these programs shine when used together, they each serve a different purpose. The next chapter will explain what each part does.