When I first started traveling, I had envisioned what I thought was an appropriate way to get from airport to my hotel.  However once I arrived at the airport, I was immediately confused by all of the options available from taxis, buses, shuttles, limos, rental cars, etc.

In fact, shuttles alone had multiple categories, including hotel shuttles, commercial shuttles, private shuttles, public shuttles, shared-ride shuttles, and park-and-ride shuttles.

In a nutshell, ground transportation is any form of transportation that takes you from point A to point B without using an airplane.  Though typically the terminology is referring to airport ground transportation methods, such as vehicles-for-hire (limos, sedans, SUVs, etc), shuttles, taxis, and charter buses.

Broadly speaking, you could also include other means of public transit including trains and even rental cars.  But for the most part, when the term is used, it is referring to limos, shuttles, or taxis that can be hired to get you from the airport to and from your hotel.