The major difference between the LSMW and cATT 

LSMW: is uesd to migration of data from non-sap system to
the sap system or sap system to sap system.here what happend
means  the data is migrated from the one system to another
system. so this is a migration tool
whereas in CATT is used to computer aided testing tool is
used to test the abap object

so this is a testing tool (catt)
this is a migration tool (lsmw)