Popocatépetl is currently in the process of a light erutpion and as a result its alert status has been raised to Yellow Stage 3, which is the third highest alert status. The image below from one of the webcams is at 12:15pm GMT, with the plume just being caught by the sun you can see that the plume seems quite white in nature, which indicates that there is a large amount of steam in the plume itself.
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.