The Cor Anglais is a type of oboe and thus of the shawm family. It is a flared pipe (with a gradual flare) with valved holes used for changing the length of the pipe to output different frequencies (pitches). The main modifier that creates a sound different (more mellow) to that of the oboe is the bulb-shape of the bell, while differences in the reed also play a part.
The acoustic principles of the English horn (cor anglais) and the French horn are quite different and should not be confused because of their names.
Source:
http://en.wikipedia.org/wiki/File:English_Horn_picture.jpg
The instrument has a deceiving name as it originated in the geographical area of modern Poland. In Middle High German it was called engellisches Horn (i.e. angelic horn). The word engellisch was also used for the ancient Angels or Angles tribe (as in Anglo-Saxon), causing a bit of confusion, hence its modern name.
The Cor Anglais was invented in the early 1700s by the Weigel family of Breslau by adding the bulb-shaped bell to an oboe. The reed (a piece of cane folded in two) of the Cor Anglais is wider and longer than that of the oboe, and has a wire at the base which holds the two blades of cane together for a constant pitch.