Some material taken from GW website. No challenge to copyrights intended.
Background: Bretonnia is one of the greatest realms in the Old World, rivaling even the Empire in size, wealth, and power. Through generations of continuous war against a horde of different foes and invasions, the Bretonnians forged a formidable and heroic tradition of Knighthood. Indeed, it is not the fertile plains and picturesque valleys that characterize Bretonnia, rather it is the proud and chivalrous Knights! There is no sight like a Bretonnian Army on the fields of battle. Multi-colored banners flap in the wind, the sun reflects brightly off the polished armor, and all is arrayed in the hues and heraldry of the many feudal dukes and lords.
What will an average army consist of?: The core of every Bretonnian army is its knights. Each army must consist of at least 50% knightly units, which can be selected from a wide variety of knightly orders. This means that a Bretonnian general will usually not have a wide variety of troops. Bretonnian armies can also spend 50% (instead of the usual 25%) on characters. This represents the greater number of heroic individuals in Bretonnia.
What are the strengths of Bretonnia?: The Bretonnian knights have few equals when charging thanks to their lance formation and powerful characters. Bretonnians can also pray before a battle, making it very difficult for the enemy to shoot or use warmachines against them. Finally, the knightly orders all come with some very useful knightly virtues.
What are the weaknesses of Bretonnia?: The expression "all your eggs in one basket" usually applies. Most Brettonian generals will have one very strong unit of knights and characters. Coupled with the high points cost and consequent small size of a Bretonnian force this means that a single powerful attack or spell can cripple or wipe out the Bretonnians. Bretonnians have no warmachines and their archers are generally considered to be rather useless (at least IMO).
Should I select the Bretonnians as my army?: Yes, if you like the idea of having your battle decided on the strength of one glorious charge. Yes, if you like having more heroic characters and fewer rank and file troops. Finally, yes if you don't mind painting tiny crests on all those damn suits of armour!! :)