A package unit puts the whole air conditioner in one cabinet: the compressor, the outdoor coil, the indoor coil, the blower and the controls. It sits outside on a concrete pad, a wall stand or the roof, and the supply and return ducts run from the cabinet into the house or building.
You'll find them where there's no good place for an indoor air handler: older homes built without an equipment closet, manufactured homes, additions, and many small commercial buildings with flat roofs. Some package units are cooling-only with electric heat strips; others are heat pumps.
The upside is simplicity. Nothing about the system lives inside, so there's no closet drain pan to overflow into a hallway and every component can be reached without entering the house. The trade-off is exposure. The entire system, including the parts a split system keeps indoors, sits in the sun, rain and wind, and near the coast it takes salt air too. That's one reason package units tend to need replacement sooner than the indoor half of a split system.
Most of a package-unit replacement is about connections. The new cabinet rarely matches the old one's footprint or duct openings exactly, so the transitions to the existing supply and return need to be built and sealed properly; a leaky transition outdoors pulls in hot, humid air and wastes the efficiency you just paid for. On a roof, the curb and flashing are part of the job. Everywhere in Broward, the unit has to be anchored to an approved tie-down detail, because the county sits in the High-Velocity Hurricane Zone.
Can a package unit be swapped for a split system instead? Sometimes, if there's a place for an air handler indoors and the ducts can be re-routed to it. It's worth asking about when the existing ductwork is in poor shape anyway, but it's a bigger project than a like-for-like replacement, and it doesn't make sense just for the sake of change. If the ducts themselves are the problem, start with duct replacement.