Helen, the carpooling example came just off the top of my head and is perhaps not the best illustration of "rota". But for instance, if you were sharing a house or flat you might establish a cleaning rota, so that everyone knew what their cleaning responsibility was in any given week. Or you could have a "duty rota", such as we had in one of the university halls I was living in, where there was one member of staff around every night, just in case of medical emergencies, floods or fires.