Generally black wants to fianchetto his King's bishop and not get it blocked in on g7, as can happen in some K.I.D.s. So black plays a very early c5 and if white advances d4-d5, black will attack the advanced pawn with e6 before white is able to defend it with e4. To achieve this black usually plays Nf6 first. After ...ed and cd, black has achieved an unbalanced pawn structure where black has the q-side majority supported by an active bishop on g7 and white has a central pawn majority which he will usually try to push before black is fully developed. Black has a slight problem getting his q-side pieces developed in many lines but the game tends to be very tactical with black relying on surprise tactics to complete his development and to achieve equality or the better game.
This is the Modern Benoni. There are other Benonis, such as one where black tries to sacrifice a pawn on b4 for queenside pressure, called the Benko or Volga Gambit, and one where black plays pawns to c5, d6 and e5 to try to block white's attack. That one's called the Boring Benoni and it gives white pressure with little recompense for black. 
Benoni Defense
I did some searching around, and I really liked the Benko, if you could explain the ideas behind that, I would appreciate it.
Oh come on, Optimissed, Czech Benoni is not so bad. GM Ben Finegold has employed it with good results and shared those game in St Loius Chess Club videos. The method of play he shows has been used by top players Anand and Nisipeanu, etc... White gets a space advantage but the center is completely closed and there is actually a lot of room for creativity. In addition to the Finegold videos Andrew Martin did an ABCs video on it a while back and the repertoire book How to Play Against 1 d4 by Palliser is built around it as the main weapon. Palliser does include a chapter on the Old Benoni move order when he says Black fails to equalize, but in the main move order he says nothing of the kind. Some players might find closed positions to be boring but I personally find slow maneuvering to be anything but and have had good results playing that way (I never made it much above 1900 consistently on Lichess but I had a plus record playing against higher rated players in that line so I think it was a good score compared to other openings.) It might not be to everyone's taste (it's completely different from what most players are looking for in a Benoni) but it's not as if there's anything wrong with it. ... I don't want to sidestep the topic, please talk about the Benko Gambit. (I just don't want people coming away from this thinking there's something wrong with e5.)
I don't play the Benko as black but I do (try to) play the Modern Benoni from both sides. However, in the Benko black aims all his pieces at white's queenside and is happy to swap off queens. Very often black regains the pawn and maintains some pressure so it's white who is trying to save the game. But there are ways white can play against it by building a kind of fortress preferably with a pawn on a4 supporting a knight on b5, if all the move order alternatives work out ok. I think the Benko is fine for black but I think it's a little bit too passive, very like the Budapest Defence in that respect. Also, white will often try to break through in the centre, perhaps by sacrificing a pawn. I generally get good results with that, even when I don't quite get it right. Benko players can turn into rabbits in the headlights.
As for the Czech Benoni, I had a chess friend who played it so I got a lot of practice against it. After trying various things I discovered for myself an approach which worked so well that the friend gave up playing it. It's simply Bd3 and Nge2, aiming for f2-f4-f5. If I get it, although it's rare these days, I simply reach my desired set-up and then start to think exactly when the f2-f4 break will be most effective. White has lots of flexibility and attacking options on the king's side and the queen's side takes care of itself, although if necessary, white can switch and prepare b4. This follows the general philosophy, taught by GMs of a previous generation, that white should always attempt f4 before b4 with this pawn structure. It is quite correct.
If your friend was allowing f4-f5 it's no wonder they had a bad time. Black can play Ng6 before castling and meet an early f4 with Ne5. If no early f4, Black can then play h6, Nh7, Bg5 to swap off the Bishop and also keep f4 under wraps, or Nh5-f4 with or without h5-h4 first. If Black isn't pushing the h-pawn then White can get f4 without giving up e5 if they support it with g3, but Black can do the same thing with g6 and f5 and the pawns just face each other and I don't tend to want capture first. If White captures I end up with hanging pawns but they control a lot of central squares and I usually do well in that position. I used to have more trouble against the b4 break, though, but in looking at master games no one took on c5 with their b4 like all my opponents did so there must be something wrong with it which I didn't figure out. It's also not as if Black can't play for b5 on the Queenside too. Anyway, yes both sides have those pawn breaks they want to work on but the maneuvering gets complicated, plans have counters and new plans. I've already posted some sources if people want to look into it.
Maybe the main problem of Benoni is that you might do everything right and still lose since understanding of the position is not enough. Concrete calculation decides if a plan or counter-plan works or not and tactics appear out of nowhere.
That's why Kaissa invented the Volga/ Benko (actually not sure it was her holiness in person, I wasn't there).
These two openings may have a similar structure, but their main ideas are fundementally different. The Benko is much calmer, although the modern treatment of the mainline is rather sharp, and requires a bit of memorization.
Thank you @pfren for the book suggestion I will surely look at it. I play more comfortably in quiet positions, so I would assume that trying to get a Benko would be better from what all of you are saying, is this right or wrong? In other words which opening is more positional.
If what you want a quiet position then I think you might actually consider playing e5, despite some other opinions. Here is one Finegold's videos I mentioned mostly looking at his games in the Czech Benoni if do want to check it out: https://www.youtube.com/watch?v=049NnoR1LGE
The Benko is a fantastic opening, lots of fun, but I believe the main line has Black under a bit of a cloud.
The Modern approach is to delay recapturing on a6 for as long as possible, which leads to some murky waters and isn't in the same spirit of the traditional endgame gambit that the Benko is often seen as.
<<If your friend was allowing f4-f5 it's no wonder they had a bad time>>
He tried preventing it. I still obtained advantages, Like I said, white first tries to get the advantage by threatening or playing f4 and then b4. White has a flexible game and I found it was easy to play when I was playing well. No-one wants to play it against me now.
If I delay capturing on a6 in the Benko, won't my bishop be trapped allowing white to play e3 and his white bishop out?
A sample line of the modern Benko goes like this:
Developing the light-square Bishop generally isn't a problem in the fully accepted lines. That said, White has a number of ways to play against this set-up (early f4, pushing a7, fast e5 pushes, a kingside fianchetto, to name a few), and general principles alone can get you in trouble.
Practically speaking, I've played several dozen blitz games with the Benko, and the vast majority either decline it, play something unambitious or fall into the Bd3 trap noted above. I haven't seen a single mainline, which is good, as I only vaguely know them.
Could someone explain the basic ideas of the Benoni, I am considering playing it as black, but I am unsure of the ideas behind the Benoni. Any help is welcome.