One of the most difficult problems in philosophy, is the nature of knowledge. There are so many conflicting views on how we even gain our knowledge that it’s impossible to tell just what knowledge even is.

The "common-sense" view of the nature of knowledge is the belief that everything our senses pick up is real in that state.  Anything we can touch, see, hear, smell, or taste is considered a factual object, anything else can’t exist.  In short, what we sense exists.

Anyone with complete faith in the "common-sense" theory instantly has a major problem: how does anyone know that oxygen exists?  No one can see oxygen being that it’s transparent, the element itself has no scent or touch, and we hear the wind passing by, not the oxygen.  We simply assume that this element exists, after all, we have to be breathing something, right?

Subjectivism - see answer 2 for description.  This is significant to the world of philosophy because, along with objectivism, it emphasizes how we as humans use our senses to understand things.

Epistemological Idealism - a sort of follow-up to subjectivism, in that it tells us that what we see and perceive consists of our ideas of what we perceive.  Thus, if someone heard a sound they’d never heard before, they’d start thinking of all other sounds similar that they had heard, and try to come up with an idea of what this particular thing could be making the noise.

Objectivism - An alternate view to subjectivism, objectivists say that things exist outside of human perspective.  Instead of relying on our senses, it relies on the object itself.

Epistemological Realism - objects exist whether or not we perceive them.  We do not make something exist by sensing it, it’s already there whether we "know" it or not.

Phenomenalism - true knowledge is unattainable, we only know what we perceive and conceive as real.

Epistemological Dualism - it’s possible to sum things up into two things, mind and matter.  Everything, eventually, comes to one of those two things.

Egocentric Predicament - placing ideals and principals into categories based on how the individual sees them from his/her own point of view.