An Ion tail is just that. It is composed of individual atoms of matter which have had some electrons removed. Do you know what an ion is?

Just in case you are not sure here is an explanation. Take an atom, any atom of any element. It is made up of two parts, a positively charged nucleus containing protons and almost always some neutrons. The protons carry the positive charge, neutrons have no charge. Surrounding this nucleus is a cloud of negatively charged electrons. In normal matter the number of protrons and electron are equal and the overall charge of the atom is zero. With an ion, the atom has either too few electrons (therefore positively charged) or too many (negatively charged). When you add or remove an electron you ionize the atom.

So in a comet, the ion tail is made up of atoms that have electrons that have been added or removed (actually most of them are in the removed state). This mostly caused by a combination of the sunlight and the solar wind striking the surface of the comet and causing atoms to get knocked off, losing some electrons in the process. The ions are mostly oxygen and hydrogen with some carbon, nitrogen and sulfur mixed in. There are also some free electons present as well.

What causes the glow? There are 2 reasons.
1) The ions interact with the solar wind and have more electrons knocked off or have electrons added. These processes generate light. (ever seen movies of auroras on Discovery channel? Same process)
2) The free electrons in the ion tail can recombine with the ions to form normal atoms again. This also gives off light.
The blue color is due to oxygen ions reacting as descibed above.

Hope this helps you understand.