Grammatical Evolution is a new kind of evolutionary algorithm that evolves computer programs in an indirect manner . The evolved "genotype" is a string of numbers (in contrast to the explicit program code genotype of Genetic Programming, for example). These numbers are decoded into a program by using them to index into a BNF description of the program's grammar, and using the indexed productions as the components of the generated program.
This book, written by GE's inventors, explains the process. There is some biological background, a description of the GE algorithm itself, and some simple case-studies demonstrating its effectiveness. If you want a brief, clear overview of GE, and some background material, here it is.