It varies from yacht to yacht...
Yachts charters vary in what is included. Additional to the rental of the yacht itself, most charters include: all standard equipment, dinghy, GPS system with charts, safety equipment (as required by the local/national authorities, bedding, towels, and berthing for you first and last night at the yacht's home port.
This is the very basics, check with your charter advisor to see what's included in your charter.