LSMW is used to data migration from non sap systems to the 
sap sytems. and also used for the mapping the fields one to 
one.three importnat points in LSMW.
1.read the data from legacy systems.
2.convert the data from source to target format.
3.import the data using the batch input,idocs,bapi's

transaction code is : LSMW
in this lsmw give the project name, sub project name, 
object name.
after giving all the projects names it will disply the 14 
steps. these are filled manually by the user.

1.maintain the object attributes
2.maintain source sturcture.
3.maintain source field.
4.maintain structure ralation.
5.maintain field mapping and conversion rules.
6.maintain user transactions.
7.specify files
8.assing files
9.read data.
10.display data.
11.convert data.
12.display and convert data.
13.create batch input.
14.run the batch input.

As told by venkat we have the following steps in LSMW, 
but we write the code in LSMW in the 

5.maintain field mapping and conversion rules."

maintain field mapping and conversion rules.

IN FIELDMAPPING WE WRITE THE CODE