The King's Indian (KI) comes in and out of fashion at the Super GM level, but throughout history it's mainly been in. Like the Najdorf, both Fischer and Kasparov championed its cause, and it was widely played during their eras. More recently it has required some revival, and for that Radjabov gets much of the credit. At the amateur level, it is safe to assume that the opening is always viable. The basic starting position is typically reached from the first diagram, but if you've looked more than casually at the defenses to 1.d4, you know that this isn't quite the starting point of the KI. Another major opening springs from this position: the Gruenfeld, which is marked by 3...d5. The two openings are distinctly different and it's the rare player who can switch from one to the other without a wholesale change in repertoire. (Kasparov is an exception.) The reason to play either opening, however, is largely the same. 1...Nf6 unbalances the position quickly, and results in increased opportunities for Black to play for the win. In general 1...d5-based systems are more solid than those beginning with 1...Nf6, but the price is fewer wins with the black pieces. In several of these KI variations, for example, Black is going to put everything into a kingside attack, and that's one of the reasons the KI is, and has remained, popular. The KI typically begins 1.d4 Nf6 2.c4 g6 3.Nc3 (top diagram)
First, it's important to note that the KI is at its core more a system than a concrete opening. Theory in the opening, however, has advanced with such breadth and depth that the KI will still feel like a complex opening that has to be learned in detail compared to some other system-based openings, such as the London System. White has a variety of ways to respond to Black's opening moves, largely because Black doesn't challenge White's control of the center for several moves and thus does nothing forcing. This results in White having much lattitude. In addition to 3.Nc3, White will also often play 3.Nf3 or 3.g3. We'll return to these two moves later in "sidelines" and Fianchetto Variation posts. After 3.Nc3, Black responds with 3...Bg7, again doing nothing to force White's reply. And now we have the KI on the board. 
White will usually respond in one of three ways. First and foremost, While will occupy the center with 4.e4. This opportunity to establish the e-, d-, and c-pawns in this unchallenged way (see diagram at left) is rare and will usually be seized by White. The major alternatives are 4.Nf3 and 4.g3. More later on these. After 4.e4, Black responds 4...d6. The d-pawn most importantly secures and supports the e5 and c5 squares, as well as makes luft for the Knights and light-squared Bishop. (One of the features that makes the KI a "system," is that it's unclear where Black's best break will come--e5 or c5?) Black can and sometimes does castle first, but note that after 4....O-O, White has the option of playing 5.e5. While White has better moves available than e5, this specific move order invites a complexity that Black had better be prepared to handle. Back to 4.d6, note that White has a large space advantage (don't set out with the KI if that's bothersome), and a superb center. The price is a slight lag in development and a weak d4 square (a square that can no longer be supported by pawns). And we've arrived at the point where White's next move will characterize and distinguish the mainlines of the KI. Note that it is White who's going to largely decide which of these major lines shape the rest of the game. This isn't surprising. Because of the hypermodern nature of the KI, Black has done little to constrain White's choices, and White has four substantial options at the fifth move.
(In addition to sidelines, we'll look at a fifth mainline in the KI --the Fianchetto Variation--in a separate post, which largely arises out of 3.Nf3 rather than 3.Nc3.)
We'll briefly introduce each of the four mainlines and then explain them separately in future posts (click on the bolded title to be taken to a slightly more in-depth overview):
1. The Classical KI: 1. d4 Nf6 2. c4 g6 3. Nc3 Bg7 4. e4 d6 5. Nf3 O-O 6. Be2 e5 *
As you might expect, there are many lines that will play out from the "Classical KI," including the Petrosian System, the Mar del Plata Variation, and the Exchange Variation.
2. The Four Pawns Attack: 1.d4 Nf6 2.c4 g6 3.Nc3 Bg7 4.e4 d6 5.f4 0-0 6.Nf3
This is an ultra-aggressive approach that has largely been de-fanged at the upper echelons, but which is popular at the amateur level. How could it not be?
3. The Sämisch Variation: 1. d4 Nf6 2. c4 g6 3. Nc3 Bg7 4. e4 d6 5. f3 *
Also aggressive, and the subject of a Super GM game as recently as yesterday, which will be included in the next post. See Ponomariov v. Radjabov in the King's Tournament in Medias, Romania, 6/17/10.
4. The Averbakh Variation: 1. d4 Nf6 2. c4 g6 3. Nc3 Bg7 4. e4 d6 5. Be2 O-O 6. Bg5 *
And note the consequences of Black playing 6...e5 at this point in time, beginning with 7.dxe5. Take a look now, as Black has to constantly be alert to combinations in the middle of the board.... 7...dxe5 8.Qxd8 Rxd8 9.Nd5, and Black has nothing better than to give up the exchange with Nxd5. The Averbakh Variation thus prevents e5 for a time.
So, the "mental structure" is in place for assimilating the KI. To summarize, we'll begin with
(a) the four major variations flowing out of 3.Nc3 d6 4.e4,
(1) Classical,
(2) Four Pawns,
(3) Sämisch, and
(4) Averbakh, then
(b) the major line of the Fianchetto Variation, followed by
(c) the "sidelines," which I'll divide into
(1) major and
(2) minor sidelines.
