The Mexican volcanic belt stretches nearly east-west across southern Mexico and is roughly parallel to the trench which lies off the SW coast of Mexico. Popo has been having minor eruptions for the last several years (2008 saw only occasional ash and steam), and is the second highest volcano in Mexico and lies near the east end of the belt. Colima is near the western end of the belt and has been erupting recently, perhaps connected to the large quake that struck that area during January (2003). The Pacific Plate is being subducted beneath Mexico and magmas caused by melting of the subducted material (seafloor rocks, sediments, water) rises into and through the crust causing the formation of volcanoes.
Here is a link to the Volcano World website.
And here is a beautiful aerial photograph of the new laval dome growing within the crater.
This perspective view was constructed looking down at a 30° angle, north is to the top. The various shades of blue and purple indicate increasing water depth.