Protein kinases are enzymes that play a crucial role in cellular signaling by adding phosphate groups to specific proteins. This process, known as phosphorylation, can activate or deactivate the target proteins, thereby influencing various cellular functions such as growth, metabolism, and apoptosis. There are two main types of protein kinases: serine/threonine kinases, which phosphorylate the amino acids serine or threonine, and tyrosine kinases, which target the amino acid tyrosine. These enzymes are essential for regulating many biological processes and are often implicated in diseases, including cancer, making them important targets for drug development.