A double overhead camshaft (also called DOHC, dual overhead cam, or twin cam (TC)) valve train layout is characterized by two camshafts being located within the cylinder head, one for inlet and one for exhaust valves. Typical automotive engines equipped with dual overhead camshafts can have from two to four camshafts in total, depending on the engine configuration. The term Twin Cam doesn't tell the exact location of camshafts, but it is mostly used to describe DOHC structure.
Double overhead camshafts are not required in order to have multiple inlet or exhaust valves, but are necessary for more than two valves that are directly actuated (though still usually via tappets). Not all DOHC engines are multivalve engines — DOHC was common in two valve per cylinder heads for decades before multivalve heads appeared. Today, however, DOHC is synonymous with multi-valve heads since almost all DOHC engines have between three and five valves per cylinder. WIKIPEDIA
