ETL stands for Extract – Transform – Load. It is a 3 step process followed for database migration, update etc. briefly the 3 steps are as follows:

• Extract: The first part of an ETL process involves extracting the data from the source systems. Most data warehousing projects consolidate data from different source systems. Each separate system may also use a different data organization/format.
• Transform: The transform stage applies a series of rules or functions to the extracted data from the source to derive the data for loading into the end target. Some data sources will require very little or even no manipulation of data.
• Load: The load phase loads the data into the end target, usually the data warehouse (DW). Depending on the requirements of the organization, this process varies widely.