Stop and frisk is when police temporarily detain somebody and pat down their outer clothing when there are specific articulable facts leading a reasonable police officer to believe a person is armed and dangerous. It is not necessary for the officer to articulate or identify a specific crime they think is being committed, only that a set of factual circumstances exist that would lead a reasonable officer to have a reasonable suspicion that criminal activity is occurring. Reasonable suspicion is one step below probable cause and one step above a hunch.
A "frisk" by definition is a type of search that requires a lawful stop. It is best thought of as a separate act, but in practice, a suspect who refuses to answer questions in a stop may be providing the officer with sufficient justification to frisk. A frisk should not be for anything other than a dangerous weapon or contraband. However, if other evidence, like a suspected drug container, is felt, it can be seized by the officer under the "plain feel" doctrine. The test for plain feel is that the item's contraband nature be "immediately apparent".