Ground staff are responsible for all the so-called "above wing" functions up to and including boarding of the aircraft. This includes, but is not limited to: passenger check-in, baggage acceptance, seating assignment and resolution, passenger manifesting, boarding of the aircraft and other customer service functions.
Generally the "below wing" functions are done by a work group known as "ramp".. they do all the baggage & cargo loading, unloading and receipt & dispatch of the aircraft. Separate groups generally do the fueling, lavatory service and catering functions.
However, each airline sets up their operation differently given how their labor groups are organized and the airport in which they operate.. Employees at smaller "line" stations tend to do a wider range of tasks than that of an employee at a larger "hub" station.