Most periods are measured in seconds. A typical windswell will have a period of 5 to 9 seconds. Whereas, a groundswell will have a much greater period from 10 upwards of 20 seconds. From a practical standpoint, a groundswell is created by a storm far away from the beach where its generated swell is breaking. It is the distance between the storm and the beach that allows the swell to organize into a long period swell.
Wind swells a generated by local storms, close to the beach at which they break.