Imagine this: people come over to your house, while you're still living there, and decide to settle down. Permanently. They rearrange your furniture, force you to cook for them; they even tell you to say things like "'wicked"' instead of "'hella."' You'd ask them politely to leave, but every time you raise the subject they shush you and remind you they've got guns. Big guns.
Then, finally, things start looking up and you push the intruders out, guns and all. Byee! But wait…they've been over for so long that your house no longer feels like your home. Worse yet—you might not even remember how things were before these folks came in. Or, you might even like how these people rearranged your home—and life!
Complicated, right? You'd think you'd feel footloose and fancy-free, and maybe you kind of do, but there's all this anger, frustration, and confusion, too. Plus, where'd they put the cookie jar?
Okay, so take that tangle of complicated feelings and apply it all to an entire nation. And then multiply that nation by all the nations that have been "'settled"' or colonized by other nations (usually the UK, but there are some other culprits too)—and voila! That's postcolonialism for you. It's all about the anguish of the colonized who have to deal with the aftermath of colonization.
As far as the theory goes, it's also about a select group of academics who immigrated from some of those nations, settled into a new country (mostly the US and sometimes the UK, but there are a few other hot spots too), and started this whole business of dissecting English (etc.) literature for its colonizing impulses.
And the select of that selection are Edward Said and Gayatri Spivak. These two are kind of like the king and queen of "'poco"' land, if only they actually believed in things like royalty and rulers (they don't. That would be totally anti-poco).
In general, they and their co-poco cohort (say that five times fast) are dead set against the whole idea of a ruling nation; they're all about resisting dominating countries and their art, literature, films, you name it, that go along with their imperialist ideologies.
Feeling politically oppressed? You've come to the right place.
You know how you always wanted to dislike that big classic novel you read in class, but you felt like you weren't supposed to because it's a classic? Postcolonial theory gives you total permission to bash that novel.
"Sweet!" you're thinking, but hold your intellectual horses because reading a novel (or anything) postcolonially doesn't mean you get to hate a book just because you think it's boring. Being a poco theory-head means uncovering all the ways that a piece of literature ( especially a classic British novel) squashes the little (wo)man who's been colonized by some big, bad, superpower nation. If you're into underdogs, then this is the field for you.
Once you get deeper into the poco theory though, you'll also start viewing literature not just as stuff to hate or like. Literature becomes a product of the real world. That means reading and teaching the stuff has real-world consequences, both good and bad.
Take, for example, William Wordsworth's poem "'I wandered lonely as a Cloud."' The poem, which features daffodils, used to be taught in places like India and the West Indies, even though daffodils don't even exist in those countries. Kind of like when you're forced to read something like The Canterbury Tales even if you're an iPad-carrying, skateboard-riding, 21st-century vegan San Franciscan. Not to say The Canterbury Tales isn't actually a pretty neat book, even if you're a card-carrying (or skateboard-carrying) postcolonial.
So if you're wondering, "'Why is this book/poem/whatever so important?"', you might want to think about how a postcolonialist might view the whole thing. There's a pretty good chance you'll find a pretty satisfying, "'real world"' answer.
Poco theory basically ties together a lot of other theories—like deconstruction, psychoanalysis, New Historicism—into a big, globalized bow. If some of those other fields have a tough time finding some kind of contemporary, "'real"' relevance, postcolonialism gives those fields a social and political urgency.
For that reason, these days, you can't not know about postcolonialism (especially its bigwigs). It gives you more than just some cultural cache; it gives you a social purpose for all that philosophizin' that you're doing.
Plus, if you're thinking colonialism is sooo last season, think again. Postcolonialism is one of the few theoretical fields that totally applies to various ongoing global struggles. Like the relationship between Israel and the rest of the Middle East (but especially Palestine), or the way a bunch of superpower nations (like Russia and China, not to mention the US) have been grabbing land in places like Africa and Southeast Asia. Pretty much take your pick of current events and ancient novels—it's all up for grabs in poco-land.