  This chapter is written for the forthcoming book "A Concise Encyclopedia of
Coding Theory" (CRC press), edited by W. Cary Huffman, Jon-Lark Kim, and
Patrick Sol\'e. This book will collect short but foundational articles,
emphasizing definitions, examples, exhaustive references, and basic facts on
the model of the Handbook of Finite Fields. The target audience of the
Encyclopedia is upper level undergraduates and graduate students.
