Popocatépetl is North Americas second highest volcano at over 5400m and derives its name from the Aztec language, which means 'Smoking Mountain'. In general, eruptions from Popocatépetl are fairly regular with around 10 eruptive episodes (which can span for many years) over the past 100 years.
Erik at eruptions has also posted on this upgrade of alert status and gives a useful summary of the situation so far and a little bit about eruptions at Popocatépetl. Latest reports for the eruption can be found here.