Well, there's all sorts and types. The band has its manager, Irving Azoff, who's in charge of getting them gigs, making deals with record companies (or helping set up the Eagles' own record company), all that business jazz. He typically takes a percentile of their earnings.
The road manager is a more nuts-and-bolts job. He makes sure everything runs smoothly on the road - not only the hotels and transport, but the setup at the venue, making sure the backing band is where they're supposed to be and when, all the practical issues. He is typically paid a salary.
Now, sometimes you'll hear personal assistants referred to as personal managers, which is confusing, because the overall manager also sometimes call himself a "personal manager." What a personal assistant does is make sure everything goes smoothy for their particular guy - like making sure he has the kind of wine he likes, little stuff like that. They can just help with little things, or in the case of something like Stevie Nicks, they become her go-to girl for everything from shopping for groceries to sending emails on her behalf and loading her iPod for her so she doesn't have to learn how, lol. I've seen her assistant Karen scooping her dog's poop for her! It depends. I know Glenn, Don, and Joe all have personal assistants who travel with them. I'm not sure if Tim does. Personal assistants are also typically paid a salary.
If you're a small band just starting out, you don't have any personal assistants, you take care of your own stuff on the road, and the only one of the above you employ is a manager.