Liberty work is generally considered as guiding a horse's activities from the ground without any mechanical means of contact. Ideally, it should be done outside of a small enclosure such as a round pen. While one may ride a horse that is also worked at liberty, the riding itself is not liberty work.
A good book to read on the subject is "Gallop to Freedom" by Magali Delgado and Frederic Pignon. While Delgado is primarily a dressage rider, Pignon is noted for working with several stallions at once in liberty. Understanding their approach to training horses can be quite helpful in considering how you might begin liberty work if you choose to do so.
One of Pignon's main concerns is learning to know each individual horse. He approaches training by trying to discover what interests the horse. By the way, Alois Podhajsky, who was the director of the Spanish Riding School in Vienna for over 20 years, wrote that he also used a horse's preferences to help decide what movements a horse would be trained to perform at shows.
Pignon states that he varies his approach depending on the particular horse. When working with several horses at once, he would change his actions and the intensity of his efforts to match the needs of the particular horse he was trying to direct at the moment.
Riding without tack is a different matter. Such riding generally begins by riding with saddle and bridle -- at least with bridle. The bridle, along with a snaffle bit, provides an additional means of communication to help a horse understand what his rider wants. As other means of communication such as weight shift and leg pressure become more predominant, the bit and bridle become less necessary.