Plasma membrane-associated small GTPase which cycles between an active GTP-bound and an inactive GDP-bound state. In active state binds to a variety of effector proteins to regulate cellular responses. Involved in epithelial cell polarization processes. Regulates the bipolar attachment of spindle microtubules to kinetochores before chromosome congression in metaphase. Plays a role in the extension and maintenance of the formation of thin; actin-rich surface projections called filopodia. Mediates CDC42-dependent cell migration. {ECO:0000269|PubMed:14978216; ECO:0000269|PubMed:15642749; ECO:0000269|PubMed:17038317}.