Elliptical galaxies are rounded, featureless collections of old stars that contain very little gas and dust. They come in a range of different sizes and shapes, and can appear circular (as shown here), oval or even rugby-ball shaped.
Ellipticals are classified according to how squashed they look. Circular ellipticals are classified as E0, and the more squashed an elliptical looks, the higher the number it is given, such that the most flattened ellipticals are classified as E7.
The smallest ellipticals (dwarfs) contain tens of millions of stars, whereas the largest ellipticals (giants) can contain over a trillion (1,000,000,000,000) stars. Not surprisingly, some ellipticals can be very dim and some very bright.
Elliptical galaxies tend to be very old, and formed their stars a long time ago. With the gas and dust long since gone, there is no material left to make new stars. In the absence of any young, blue stars, ellipticals appear yellow-red in colour. For some reason, elliptical galaxies are the most common type of galaxy in clusters.