Database Review: The Ethnologue
What it is: The Ethnologue is a compilation of information about almost 7,000 languages. It includes information about the relationship between various languages, the number of speakers of each language (or the date the language became extinct), and the countries in which those speakers reside.
How to use it: Start with a language and find out what family it belongs to, how many people speak it (1st and 2nd language speakers), and where it's spoken. Start with a country and find out what languages are spoken there and by how many people. Start with a language family and identify all of the subfamilies and languages in it. (You can also find information about the state of Bible translations in most of the languages since the site was started by a group of Bible translators.)
For example, I was once working on a project where we had to digitize all of the bib records in a collection, except for the ones in Indo-Iranian languages. The problem? We didn't know what languages were considered Indo-Iranian. So we pulled up that subfamily in the ethnologue and made a list of the languages we could skip.
I've also used the site to figure out how closely related Russian and Ukrainian are, and where Swedish is spoken outside of Sweden.
Pros and Cons: First, some minor quibbles. The Ethnologue assigns a 3-letter code to every language, but it's not the same set of codes assigned by ISO 639. This shouldn't affect the average person on the street, but for librarians, it means that the three letter language code you assign in a MARC record isn't the same as the codes used by the Ethnologue, which is sort of a pain.
Another issue is that linguists don't all agree on the assignment of languages to families and subfamilies, so any one presentation is going to meet with disagreement by someone, somewhere.
The biggest issue is that the Ethnologue folks don't regularly update the information presented in the website, which means that some information, such as populations of various countries or the number of people who speak a given language, is woefully out of date. Basically, it's a good place to get a general overview of a language and its relationship to other languages (and almost the only place to get such information about all the languages in the world), but if you want more current information and scholarship, try another source.