In a world where bloodthirsty, hectic combat is the norm and tempers run high, the Monks of Pathfinder provide something of a calm center. Molded from their origins to be quiet and centered, Monks are nevertheless fearless and fearful opponents - not to mention great allies, so long as you don't try to break the law too much.
Though they can use a few kinds of special weapons to great effect - kamas, shurikens and quarterstaffs - Monks typically excel and hand-to-hand, unarmed combat. The punch of a Monk gets stronger over time and is a great deal more potent than a typical Unarmed Strike, often getting stronger than most weapons. This makes a Monk a shoo-in for high Strength and Dexterity scores, though Constitution is also good for high points and Wisdom will help the Monk by providing extra AC - assuming he keeps heavy equipment off his person.
This, for many players, is the sticking point of the Monk: in order to benefit from most of their attacks and abilities, Monks can't wear armor. Consequently, players will have to look for alternate ways to boost the Monk's AC. Choosing a naturally tough creature as a base class - perhaps a Lizardfolk - is a solid idea for more AC, as well as tossing a bunch of magical protective items on the Monk. A Monk should look to get a Monk's Robe as early as possible to boost their effective Monk level and gain more AC, and he may want to multiclass with a magical class that provides spells for boosting AC, like Mage Armor.
In combat, Monks belong in the front rows. Though they're good with Shurikens Monks do the most damage right in the enemy's face. This does not mean, however, that the Monk needs to rely solely on punches for damage. Because Monks get advantages to their CMB roles from high Dexterity, Strength and class attributes, Monks are great at tripping, bullrushing and feinting enemies, especially since they can zip in from a distance with their high speed. Incapacitating an enemy is more important than simply sitting back and trading blows, since the Monk's often lower AC will get him in trouble. Choose lots of strong combat feats that complement your style of attacking, not to mention any specialized Monk path you might take.
Monks are similar to Paladins in Pathfinder in that they must retain a consistent alignment, though in the Monk's case this is any Lawful alignment - and if they lose it, they keep all their powers. They simply can't progress any further until they atone for their actions and move back to their old alignment. This can prove constricting, though Monks are much less particular about unscrupulous companions than Paladins, and thus are easier to role play. Monks also get some similar benefits to Paladins, such as the ability to ignore diseases and quasi-mystical powers via their Ki points, and thus may prove a more equitable choice overall.
A Monk is not a staple of a successful party. Other classes can fill their role and do just as well. If you anticipate a lot of one-on-one battles where speed is a necessity, however, go with a Monk. Their ability to survive on little equipment makes them great choices for a team that normally likes to spend a lot on arms and armor.