A linguistic map is a map that shows the geographical distribution of speakers of certain languages or regional dialects. While in the USA the main languages would obviously be English and Spanish, in areas of the world with diverse languages like south Asia, linguistic maps can very very interesting and varied.
Here's a linguistic map of that very area, thanks to Columbia University's linguistics faculty (click on the map for the full-size version):
India is famed for its incredibly diverse linguistic landscape, and this map shows that its fame is well-deserved.
As with the etymology map posted a while back, this map seems alarmingly complex at first, but the closer you dive into the map, the more you realize that it's simply listing regional languages and dialects and showing which family each language branch belongs to.
If you're interested in seeing more linguistic maps, there are plenty available on muturzikin.com, including many linguistic maps of the USA.