The bard is perhaps the ultimate generalist. In most adventuring groups, he works best in a supporting role. He can’t usually match the stealth of the ranger or the rogue, the spellcasting power of the cleric or the wizard, or the combat prowess of the barbarian or the fighter. However, he makes all the other characters better at what they do, and he can often fill in for another character when needed. For a typical group of four characters, the bard is perhaps the most useful fifth character to consider adding, and he can make a great team leader.
The bard is among the most versatile classes, with great skills, deceptively good spellcasting, and decent combat abilities, the Bard can fill nearly any role in a party without depending entirely on spells to do so like the Wizard does.
A classic criticism of bards it that they are a "jack of all trades and a master of none." Well, the full line is "Jack of all trades, master of none, but often times better than a master of one", and I can think of few things to which that truism applies better than to the Bard.
The bard's biggest roles are as a Face, Librarian, Support, and Utility Caster, though they can fill nearly any role with a handful of clever build choices.
Unfortunately, due to their broad skill set bards also suffer from a little bit of MAD. They're not monks, but it can be a problem. Where you choose to put your best ability scores will define the function of your bard, but there are surprisingly few bad choices.
Str:(☆☆) If you plan to fight at range with a crossbow or with spells, you can dump Strength. Otherwise, you may want decent strength for a melee weapon or just a little bit for bonus damage on a composite bow.
Dex:(☆☆) Bards only get light armor (medium with Battle Caster), so you'll want some Dexterity to boost your AC. If you go for ranged weapons or weapon finesse, you'll want a bit more.
Con:(☆☆☆) With only 1d6 hit points and poor Will saves you'll really need Constitution.
Int:(☆☆☆) Bards get too many skills, and 6+ skill ranks just isn't enough without some Intelligence to back it up.
Wis:(☆) Bards have good Will saves and very few skills tied to Wisdom, so this is your only true dump stat.
Cha:(☆☆☆☆) The Bard's most important abilities are powered by Charisma, including spells and Inspire Courage. However, since the Bard is a little bit MAD and you're not a full cast, you don't need to start with and 18 at level 1. It helps, but it's not required.