The 900 and 1800MHz bands are the only bands used in the European Union, and in fact in most of Europe. They are also the prevalent frequencies used in Asia and Africa.

To my knowledge, GSM is the only protocol that runs in these bands. The European Union mandates that all mobile phone networks in its member countries must run GSM, and the 900 and 1800MHz bands were already in use in Europe, so 900MHz GSM and 1800MHz GSM equipment was readily available to be deployed in other countries as well.

The 900MHz band was introduced before the 1800MHz band, much like the Cellular and PCS bands in the United States. For the same reasons that Cellular (800MHz) coverage tends to be better than PCS coverage, coverage in the 900MHz band in countries that use it is generally better than in the 1800MHz band, although in many densely populated European countries, both frequencies have very good coverage.