In biochemistry, isozymes (also known as isoenzymes or more generally as multiple forms of enzymes) are enzymes that differ in amino acid sequence but catalyze the same chemical reaction.  Isozymes usually have different kinetic parameters (e.g. different K values), or are regulated differently.  They permit the fine-tuning of metabolism to meet the particular needs of a given tissue or developmental stage.
In many cases, isozymes are encoded by homologous genes that have diverged over time.  Strictly speaking, enzymes with different amino acid sequences that catalyse the same reaction are isozymes if encoded by different genes, or allozymes if encoded by different alleles of the same gene; the two terms are often used interchangeably.