Marshal is an old loanword meaning "Stable Keeper" that has developed through the ages to represent someone who rounds up other people.
This has been used in military contexts to represent field commanders, or in law enforcement contexts to represent agents of the courts and/or various varieties of bailiffs.
Instead they use the rank of Marshal as their closest equivalent for the commanders of their individual crusade fleets.
The Adeptus Arbites includes the rank of "Suffering Marshal", which is a playable career in Dark Heresy, representing those officers who go on detached duties in order to track down and recover Imperial fugitives, usually from worlds which are outside of Imperial jurisdictions.
3rd Edition D&D
Printed in the Miniatures Handbook, like many of the classes presented there, the Marshal is optimised purely for battlefield combat and doesn't perform really well compared to other PC classes.
As expected from professional soldiers, they are trained in all armour and in simple/martial weapons. However their base attack bonus is the same as Rogues and Clerics, and their Hit Die is decidedly average at D8 so they do not really count as front-line warriors.
What they do get in terms of class abilities however is access to team enhancing abilities: First in the form of Auras which will be explained in further detail below, as well as the ability to grant ALL of his allies within 30' an extra move action a number of times per day.
The Auras come in two formats, Minor or Major, and you can have one of each active at the same time. Each Aura grants circumstance bonuses to the marshal's allies within 60' as long as they remain conscious, mobile and audible and the magnitude of these bonuses scale with the Marshal's level.
Minor Auras generally provide bonuses on characteristic checks, which often require the PC to know what he's expecting in advance to be of any use, so is therefore situational. But since the bonuses can be swapped freely it's not such a big deal.
Major Auras are far more sweeping, and grant positive modifiers to things such as attack rolls, armour class or movement speed. These are far more useful to your allies as in most cases everyone shall be able to benefit from them.
From the way the class is built it really needs to be used in much larger scales rather than small groups of PCs, otherwise the bonuses that the class provides could be more easily outstripped by a divine caster through buffing spells.
Making Marshals work
Unfortunately, there are limited options available to Marshal PCs in terms of alternate routes to take. Very few feats or prestige classes really improve their core abilities in any way.
So unlike spellcasters, rogues or warriors: multiclassing away from Marshal will essentially mean starting a whole new progression.
At its simplest, taking the Leadership feat works well, as having more allies in your vicinity grants the greatest return on your investment, as even a whole bunch of lv1 commoners can be made relatively accurate with your maxxed out auras, and be made to move around more quickly with your ability to grant extra actions.
There are other things to look out for if you know where to look.
Dragon Magic introduces Draconic Auras, which can be taken as a feat and used in addition to your normal auras. If your PC is of the dragonblood subtype they also scale with his level too, regardless of class. So you can make your PC the central hub of a nicely-buffed army. There is also a feat to allow your project two auras, allowing for a potential grand total of four. You just need to be careful about which bonuses you provide, because as per the basic rules, they do not stack if they are of the same type.
In the same book there is also the Dragon Lord prestige class, which provides all the Draconic Auras and allows you to project one yourself. So when combo'd with with potential feats and class abilities you could end up projecting up to five auras. It also has a full BAB progression and d10 hit die, so therefore is a good option to get Marshal's more involved in combat, rather than sitting on the sidelines cheering his party on.
The class also enhances your Leadership feat if you have it, by giving you a draconic cohort and also gives you a number of other team-enhancing abilities such as granting temporary hit points and morale bonuses to attack rolls. In addition, this class also eventually buffs you in relation to how many enemies you get flanked by.