what is flex?

[Aaliyah]

I want to know that What is flex,i mean is it an language or other?

[wvxvw]

Well, you just started to ask in the wrong forum :) Flex 1 is hardly ever used at all... it was built upon AS2 mx.* classes and that was rather a disaster :)
Now you would want to use Flex 3.2 and above.

Flex is an AS3 class library, a set of compilers and some other utilities designed for better AS3 development. These all together usually referred to as Flex SDK.
These are free and opne source.
There's the IDE which up until recently was called Flex Builder (the last one is Flash Builder though). The IDE is a commercial product.

There isn't such thing as "Flex language" or "Flex player", whatever you do in flex compiles for either of these runtimes: Flash Player or AIR. There is a plan of upgrading Flash Lite to version 3 (to support AS3), at the point this will happen Flex will also compile to this runtime.
However there is MXML language, which is a kind of macro / markup for faster AS3 coding. Right at the moment MXML can be compiled only by using Flex compiler, so, in a certain sense you can call that "Flex language". However the situation will change once Flash Catalist will be released because FC will also use MXML for layout.
If you're familiar with .NET, then MXML to AS3 is the same as XAML to C#, VB and other .NET languages.