What is the difference between being the only manager, being a part of a management team (the big M) or being a tour manager (TM)? How does a personal assistant differ from a tour manager?
If someone is the only manager of a band and follows them on tour, he probably won't have time to speak to his band at all during this trip, unless it's to usher them to the stage or bail them out of jail. His first request when they reach backstage will be an office with a phone and wireless internet, and he'll be spending his time there getting new gigs, finalizing contracts and discussing riders with venues the band's playing next month, or even next year. If he's a bad manager, or one with a way too high workload, he'll be finalizing the details of tomorrow's gig. This usually makes it hard for the venue to cooperate with him, be it for rider demands or changes in the set, as a lot of these things are finalized at least a month beforehand. Except from when he's knocking on their door when it's stage time, you won't see a sole manager touring along with the band. It's inefficient, but for beginning bands it's the only way unless one of the band members volunteers to manage every aspect of their tour (not fun, but it have been done successfully in the past, rarely by the lead member of the band, aka. lead guitar/vocals, because of ego issues as well as time issues (no one wants to interview the drummer, or bass player)).
If an artist is famous enough to warrant more than one manager, that means that a manager travelling with the band can dump some of his workload off on others while they're on tour and focus on the tour manager side of the job. The management team back home fixes the economy (they get the fees, they pay for the travel and stay and they make sure the band has everything they need). They will also be the ones to contact venues about riders and questions, and then let the tour manager or the band know what they should expect in every city. For obvious reasons (I work at the festivals) I rarely have contact with the management team. Whenever I do, it's to make them admit that they fucked up. The main problem is usually communication. The management team forgot to let the band know that the fee only covered their travels TO the festivals, they are now stuck in Norway. They forgot to let the band know that the festival is strictly non-alcoholic, refuses to get them hard spirits or cigarettes (because of a very convoluted set of laws making this too much hassle) or that you simply can't get Budweiser in Norway (because it tastes like cat piss).
A tour manager is always used to life on the road. I have yet to meet a fledging TM. Personally, I believe they grow somewhere east of London, and are harvested annually. The TM will want to be present at Get In to check that they got what they wanted on the technical rider and catering rider (check thesmokinggun.com for both kinds of riders, and this post about catering riders) and learn the layout of the venue. He'll be the one making sure everyone who should be on the guest list is on the guest list, lets security know how closed backstage is (just the band, just those with AAA-passes (access all areas, usually the festival/venue's production team, the band and the band liaison), by invite only or everyone on the guest list) and arranges lunch, interview and anything else that pops on. He will be the main contact between the venue and a band, and with a good TM the band never has to speak with the venue workers unless they wants to.
The job as a TM varies between cat herding and paperwork (if a band member is arrested, it's your job to get them out. If they disappear, it's your job to find them. If the instruments are gone, you're the one staying behind at the airport to argue with the airline until they magically reappear. If your singer is caught in customs with five grams of cocaine, you're the one to get the number to a good lawyer and/or bribe everyone in sight.) The list of what a TM does not do is a lot shorter than the one about what they do. This is why a lone manager has to be prepared to work hard.
As for a personal assistant, I find that bands rarely travel with one unless they have some special needs the TM can't cover. This can be because of different health problems (loss of hearing, social anxieties, old age, other handicaps), because the artist has side projects they need an assistant to handle while they and the TM focuses on the music or because the artist is a diva. In all cases, communication with the artist often goes from the venue to the TM to the assistant to the artist. I have very rarely worked with artists who had personal assistants, but find that their job is the same as the personal assistant of any celebrity. Mostly gruntwork, fetching and bringing, as well as keeping time for interviews and meetings and doing all the boring parts of travelling (if you travel with both a TM and an assistant, the assistant is the one who's left at the airport, looking for the lost luggage).