Cams help you manage opening and closing of air intake and exhaust valves.. Single cam has lobes that will manage both intake and exhaust, where is DOHC, allows for separate controls. Some engines do not have cams instead use lifters... some of the most successful engines in history did not have cams. Watch out of computer/electric controlled valve technology coming soon... it will eliminate cams and add very accurate valve control that will change the performance and efficiency of the engine. surely engine will be more expensive. Some of these are partially still under use in V6, V8 engines where some cylinder are shut down to increase fuel efficiency.
This is also related to timing belt/chain as cams are moved by timing belt/chain. check out how stuffworks.com and look for timing...video