Tyrosine phosphorylation is the addition of a phosphate (PO43-) group to the amino acid tyrosine on the protein. This transfer of phosphate group from ATP to the amino acid tyrosine on the protein is made possible through the enzyme tyrosine kinase. Tyrosine phosphorylation is considered to be one of the key steps in signal transduction and regulation of enzymatic activity. In tyrosine phosphorylation are important two classes of tyrosine kinase, receptor tyrosine kinase and nonreceptor tyrosine kinase. Receptor tyrosine kinases are type I transmembrane proteins possessing an N-terminal extracellular domain, which can bind activating ligands, a single transmembrane domain, and a C-terminal cytoplasmic domain that includes the catalytic domain. Nonreceptor tyrosine kinases lack a transmembrane domain, most are soluble intracellular proteins, but a subset associate with membranes via a membrane-targeting posttranslational modification, such as an N-terminal myristoyl group, and can act as the catalytic subunit for receptors that lack their own catalytic domain.

Reference
Wiki: Tyrosine Phosphorylation