ETL (an acronym for "Extract-Transform-Load") describes a data process that obtains data from some source location, transforms it, and delivers it to some output destination.
Most WPRDC ETL processes are written in rocket-etl, an ETL framework customized for use with a) CKAN and b) the specific needs and uses of the Western Pennsylvania Regional Data Center open-data portal. It has been extended to allow the use of command-line parameters to (for instance) override the source and destination locations (pulling data instead from a local file or outputting data to a file, for the convenience of testing pipelines). It can pull data from web sites, FTP servers, GIS servers that use the data.json standard, and Google Cloud storage, and can deliver data either to the CKAN datastore or the CKAN filestore. It supports CKAN's Express Loader feature to allow faster loading of large data tables.
Some WPRDC ETL processes are still in an older framework; once they're all migrated over, it will be possible to extract a catalog of all ETL processes by parsing the job parameters in the files that represent the ETL jobs.