RTL is a family of private TV stations in Germany
(though there are also RTLs in Luxemburg
(RTL does stand for something like Radio and Television
Luxemburg) and the Netherlands
, but I do not know much about them).
In Germany there are three RTLs:
RTL - the main station, is one of the most popular tv stations in Germany, has new films, talkshows and sport
RTL II = started as a station to send all the old RTL material, now it is well known for children programmes, big brother, soft-sex films and reports
Super RTL - is now what RTL II has been in the beginning, old cartoons, old films, old series and really bad music shows.