Service listings are listings for providing productions with professional tasks and duties that are performed by a person. An example of such would be a DJ or a Photographer.
Crew simply means that more than one person is included in that service listing. For example, A photographer might include a Make-up artist and a light technician as part of their service listing. These may or may not include all the necessary equipment.
If you are requesting a booking, be sure to ask what’s included if the Listing doesn’t provide all the details. If you are creating a listing, be sure to include what exactly your Service includes and what is extra.