The study of the origins of languages and their classification into families is traditionally known as philology. For various reasons it is not possible to be precise about the number of languages in the world, but most philologists agree that there are between 6,000 - 7,000 living languages. These languages are divided into about 100 language families; the exact number is dependent on the classification paradigm. The major language families can be further divided into groups of languages that are also called families. So, English belongs to the Germanic family, which in turn is part of the Indo-European family.
Clearly, in terms of second language acquisition, it will generally be easier to learn a new language from the same language family as the mother tongue than to learn one from a different language family. A German student is naturally going to have an easier time learning English than a Chinese student.
The most comprehensive and reliable information about languages and language families has been produced by the Ethnologue. The information that follows on this page has been derived from this source.
Here is a list of the 10 major language families (in terms of the number of speakers of those languages worldwide and/or the number of sub-families/languages they contain.) In each case, the language family is followed by one of its sub-families, followed by an example of a language from that sub-family.
The following list gives a breakdown of the number of native speakers of the world's major languages. The numbers themselves are rough estimates but give an accurate picture of the relative positions of the languages listed. (Sampled 13/02/2007 - all numbers in millions.)
Note: The Arabic languages are listed separately in the Ethnologue. Together it is estimated that about 220 million people are native speakers of a variety of Arabic.