Generally, as a big 'You know a legend/read a book/heard a song/heard a rumor about...' sort of thing. Fewer hard facts - this is what Knowledge is for - and more information that folks might think is just rumor, legend, or myth. This doesn't mean that bardic knowledge is always (or even often) wrong - it just isn't the information that's going to show up in academic works. As an example:
Knowledge (nobility) checks, in regards to the local baron, might reveal his lineage, policies, important members of his court, his coat of arms, etc. Gather Information or Knowledge (local) might reveal rumors about the baron and his lands. Knowledge (Geography) will sketch out the borders of his land and important landmarks. Knowledge (History) might reveal the past of the barony and how it has interacted with neighboring lands. Knowledge (religion) can tell you what the most common faiths are in the barony. And so on.
Bardic Knowledge might give you much of the same information... however, its more likely to be a smattering of factual knowledge (the baron's father was a general in the king's army), rumor (the baron claims to have routed the bandits in the woods, but he's actually paying them to attack a neighbor), and legend (the crypt under the baron's castle holds the Scepter of the Magi, but the baron is unable to wield it).
So, instead of learning a whole lot of facts with Knowledge checks (That your character *knows* to be facts, or at least information that the majority of the world he has interacted with believes to be fact), you get a variety of information that can range from highly accurate and easily verifiable by reputable sources (there's likely dozens of sources that can verify that the baron's father was a general), to claims that some view as factual and others dispute (the neighboring lands might claim the baron hired bandits to raid them, while the baron denies this) to things that, as far as most people are concerned, are myths and legends from the tales of other bards trying to earn a few coppers ('Yeah, right, that's a good one - there's a crypt under the castle, and the Scepter of the Magi is in it!'). It doesn't mean any of that information is true or untrue, just that those are the sort of stories that are out there.
Overall, an individual knowledge skill check is going to give more information, more detailed information, and more accurate information on a particular subject than a bardic knowledge check. However, to get the breadth of information that bardic knowledge has, you'd need to make a number of knowledge checks in a variety of fields, albeit at lower DCs than the bardic knowledge check might have.
Basically: Knowledge Skills = Depth and accuracy. Bardic Knowledge = Breadth, and information that isn't generally considered facts (which, in certain circumstances, *can* be more valuable and more accurate than the things that *are* considered facts, but are at least as often red herrings).
