Just In Time compiler is another important term in CLR.This 
means that take action as soon as request comes.This term 
is taken form china people.They dontkeep anything in 
inventory. So whenever a resquest for any item arises, they 
purches that item & give to requester; this operation is 
called as Just In Time.
The MSIL, which we canview using ILDASM, is passed to 
CLR,When JIT encounters call to particular method, which 
get converted into Native Code. 
There are three types of JIT Compiler.
Standard JIT
Pre-JIT
ECONJIT

JIT is a program which compiles MSIL to machine code, 
without this .net has no path to machine code.
Types:
Standard JIT: It produces highly optimized machine code and 
it caches the code in case it need to access 
again.Compilation of IL to native code is very slow however 
it produces native code very fast. To make it fast machine 
use lot of Ram.
Econo JIT:This is designed for small system which donot 
have lot of RAM.But it Produces the code which is not 
optimal and also it donot cache the code.

JIT compile the native code, only that part of code will be
compiled which is needed, rest is not compiled untill not
needed.