ETL VS ELT

what is the difference between ETL and ELT ?

ETL 
The transformations are processed by the ETL tools (Informatica , Datastage etc) while in ELT the transformations are processed by the target datasources (RDBMS). I guess that the rising scalability of databases allows this kind of data integration to be possible.
So the important thing to remember is the side where the transformations are processed.
I made the following schema to help remember the difference :

ELT 

 
This is appropriate for databases like Teradata & Netezza and this approach is the Future . The Engine which Teradata uses is efficient and Super fast at ELT (Extract, Load, Transform) data processing.
ELT should be preferred , instead of ETL, in below cases :

there are big volumes of data

the source database and the target database are the same

the database engine is well adapted for that kind of processing (ex: Teradata)