The head cowboy
The trail boss was an older cowboy (or ex-cowboy, if you like) who had experience with cattle and cattle drives. He knew the lay of the land, and determined the route taken, campsites, fording sites (preferably rock, not mud bottoms), and the speed at which the cattle were driven (fast enough to keep on schedule, but slow enough that the cattle didn’t lose too much weight). He also choose a crew (cook, horse wrangler, and a handful of cowboys), and was in charge of them while they were out on the range.
He would often ride on ahead of the rest of the crew to check the route for good water and grass for the cattle (these factors could change over time), and vary the route and speed of the herd to adjust for any problems. He was also the one who made any necessary arrangements with settlers and inspectors; and the one who paid 'bounty' to the Native Americans, who often demanded a tribute of beef when the herd passed through their lands.
The trail boss was the best paid of the crew, usually making about $100 a month, compared to $30 each for the cowboys.
I was hoping to add a few names of the more famous trail bosses, but the only one I can find much mention of is Ab Blocker. If you have others to add to the list, let me know.