yes, there are different strengths.
If you are talking about fentanyl patches, in each of them there is a resevoir of pain killing medicine that is held next to the skin by a sticky plaster typething and changed every 3 days, giving a constant background level of pain relief. Reasons to have them instead of oral morphone might be if there were swallowing difficulties, or nausea/sickness that prevented tablets being taken or if the side effects from morphine tablets were intolerable. Sometimes a patch might be more tolerable to the patient/person