Postmodernism at a glance
At the most basic level, postmodernism is the most recent philosophy that offers societal views of things like truth, community, the use of language and a lot of other things.  (Look at the end of this article for a more academic understanding by Moreland and Craig.)    Among other things, postmodernism includes the view that truth is dependent on context.  In other words, it is impossible to know anything to be true except from your own point of view.  In fact, any kind of what is called “dichotomous” thinking – that one concept can be better than another one (beautiful/ugly, good/bad, true, false), is generally rejected.  Truth, as an absolute concept, is pretty much denied.
On a positive note, postmodernism has also brought a lot of attention back to other aspects of context, like the importance of community.  I think this is very important, and has been largely set aside by the Church in the last few decades, even though it is a vital part of the Christian life!  I have been gratified to see the shift back toward community in the Church recently.
Two great Christian philosophers, Moreland and Craig offer key tenets in the way Postmodernism sees things, especially in reaction to Modernism:
“Reality” is considered a social construct.  Since language is totally contextual and reality can only be described in language, reality is a social construct.

1. Rejection of dichotomous thinking.  Basically that one thought would be “better” than another, eg. True/false, beautiful/ugly, good/bad.
2. Rejection of “foundationalism.”  The search for certainty; this tendency is often referred to as “relativity” in regards to truth.
3. Change to Nominalism.  Words do not represent universals, but are rather just names to describe.
4. Rejection of “essentialism.”  There are some traits that actually define a subject – like “roundness” for a circle – that if lost, the subject ceases to exist.
5. Community defines interpretation.  The community decide what an idea, or book means, not the author or any other “authority.”
6. Everything is a narrative.  The story of a certain group of people from their certain point of view.

(Moreland & Craig, 144-151)
Summary
In the postmodern world, we are no longer gods who are able to bring about the Utopia of the universally great society of the modern world, now we are gods only in the sense that we have no other gods before us – we set our own standards for right and wrong, fact and opinion.
In this thinking, there are no external sources for anything.  I am the only source for truth, value, insight, opinions, morality, etc that I fundamentally accept.  All others are external sources that I am under no obligation to accept.  Who is culture to tell me about my gender?  Who is genetics to tell me about sexual identity?  Who is God to tell me what is right or wrong?  Who is science to tell me about what is healthy or not?
It should be clear why, as Christians we reject both of these.  Let us take some time and look into who we are biblically:
God is the Father of all creation – our legitimate Father; our initial relationship has been broken by our sin, and we have become a part of the household of sin…but our original, legitimate Father still seeks us.  The biblical picture of Fatherhood is wrapped up in these anthropological ideas
 “Our origin lies in God” – this is an important concept, which translates into the ideas that
i.      We cannot create ourselves; we cannot choose whether or not to be.  Our life is owed to God, not us, and
ii.      We are also unable to create our own reason for being.  If it was not by our own teleology that we exist, then it is not us who has given us a purpose.
iii.      Similar to above, if God can only offer purpose, only God can prescribe value as well.   Society cannot succeed at this, nor can it be discovered by merely looking within.  If looking within worked, we should be able to make the question of purpose simple “Why did I create myself?”. This is significant issue, for example, in the discussions of the topics of abortion, euthanasia, the handicapped.  God calls good (valuable) what He calls good. – a discussion of self esteem also is part of this.
This is significant in the discussion of value.  Value can be either prescribed (based on a notion of intrinsic value) or postscribed (generally based on performance).  Generally, prescribed value is tough – it is a prediction of how valuable something will be before it is done.  I would think that usually, only inventors, artists, visionaries and creators would be involved in this often – other than parents.  Postscription value is usually based on what the greatest expert is willing to pay.  In both cases, God’s value in us is virtually without limit.  He has created each of us unique and gifted for general and specific purposes – furthermore, He (as the flawless expert) has bid the highest price payable for each of us and for our race.  In both realms, we should be pleased by the determined value we each carry with us.  At the same time, it should sober us that we have so little to do with our determining our value ourselves.
What has postmodern thinking done for Christianity lately?  One is the value of the context.  I do not think that the new and more accurate understanding of Jesus in the context of being a Jew (His context), rather than a Hellenist (our context) would have happened without some of these changes in thinking.  There is more, but that is a huge one.
“You come of the Lord Adam and the Lady Eve,” said Aslan.  “And that is both honor enough to erect the head of the poorest beggar, and shame enough to bow the shoulders of the greatest emperor on earth. 
Be content.”
                                Aslan – Prince Caspian, Chapter 15