What is the difference between the ETL and ELT?

ETL: Extract, Transform, and Load (ETL) is a process that involves extracting data from outside sources, transforming it to fit operational needs (sometimes using staging tables), then loading it into the end target database or data warehouse. This approach is reasonable as long as many different databases are involved in your data warehouse landscape. In this scenario you have to transport data from one place to another anyway, so it’s a legitimate way to do the transformation work in a separate specialized engine. 

ELT: Extract, Load, Transform (ELT) is a process where data is extracted, then loaded into a staging table in the database, transforming it where it sits in the database and then loading it into the target database or data warehouse.