Let's say you come upon a tree, and you're the first person ever to do so. You name it "tree", and tell other people that you're going to call that thing "tree" from now on, so that they know what you're talking about when you say "tree". Easy enough, right?

Next, you notice the tree has a kind of round shape, like a sphere. So you call that "sphere", and let everyone know that the shape you're talking about is a sphere.

Then, once you've figured you're doing okay naming things, along comes Socrates. He asks questions like, "Is there a perfect sphere?" which is a fun idea, and makes everyone laugh, but you can't answer, because you're not a philosopher, you were just making up names for things so you could talk about them.

But Socrates continues. He establishes that because he can imagine a perfect sphere, that it is the "essence" of the sphere, the very thing that gives a spherically-shaped object its spherical-ness! Furthermore, these vague notions that you have in your head are the only truly important things in the world.

Socrates wanders off to mutter to himself more, and you imagine that the poor bastard must have hit his head particularly hard.

See, Socrates was going in circles. The way we name things is vague in the philosophical sense, so that when Plato's teacher tells us that there are "Forms" for everything, he's saying that our mental search heuristic is actually more precise and important than the class of objects itself.

That is, we apply a word like "tree" to a bunch of characteristics, so that we can communicate. We can identify things as trees based on a few vague characteristics, such that even though two trees of the same variety aren't the same, they can still fall into the category "tree". Even different species of tree are still trees. Such is our power of categorization that we can decide on a short list of attributes what something is or is not.

But what Plato presents us with may actually be worse than circular, as the process of making forms is a string of non sequiturs.

A) Human beings identify a sphere as "sphere", therefore 

B) A perfect sphere must exist as the model for our collective notion of one

isn't even close to rational. Longer forms of the implicit argument could be shown, but that's what it boils down to.

The Platonic forms are an exaggeration of our search heuristics: lists of attributes that vaguely define a category. As such, it's pure trickery to suggest that any category or sub-category could be described "fullly", or in accordance with its "essence" or "form" or any other euphemism for this process.

It's completely backwards, so I don't know why we have to continue to discuss it philosophically. Unless we cite "tradition" as a reason to do things.