What are the reusable transforamtions?

Reusable transformations can be used in multiple mappings.
When you need to incorporate this transformation  into
maping, U add an instance of it to maping. Later if yoU
change the definition of the transformation ,all instances
of it inherit the changes. Since the instance of reusable
transforamation is a pointer to that transforamtion, U can
change the transforamation in the transformation developer,
its instances automatically reflect these  changes. This
feature can save yoU great deal of work.

 If you want to perform similar task for diff mappings, and 
if your logic is also similar for the diff requirements. in 
that case instead of creating multiple mappings, we will 
create it once and we will select the option as resuable.
so that we can reuse it in other mappings also.
but if you want to do the modifications in reusable mapping 
its not possible. you need to do it in original 
transformation it will will reflect all the other instances 
of that particular transformation