GeoKettle is a spatially-enable version of the Pentaho Data Integration Kettle software. Kettle and GeoKettle are Extract, Transform, and Load (ETL) tools.
When you have some GIS data (say in a shapefile) and you want to transfer some part of that data into a spatial database you can use GeoKettle to EXTRACT the data from the shapefile (it does it one feature at a time) apply any TRANSFORM-ation required, for example you could change the coordinate reference system, or calculate the area of a polygon, or select only some of the data based on some logic, and then LOAD it into the database. The GeoKettle environment is GUI driven, so you drag a step onto your workspace that handles a part of the process, then add further steps and link them together to form a processing chain.