An Activity is a definable step in a process. Some might call it a Task. It has a duration and may require certain things (resources) to be available in order to be scheduled.
A Resource is anything that is required to support an activity. When an activity is scheduled, it obligates its required resources for some interval of time. Resources have certain availability which is modified as activities are scheduled or unscheduled.
Example: A service call is an activity which requires resources such as a technician, a vehicle, and a repair kit. One activity, three required resources.
* Think of activities as actions, resources as the objects that are required to accomplish those actions.