There are around 12 million Czech speakers in the world today, with pockets of native Czech speakers in various parts of the planet. Many, according to Wikipedia, live in the Czech Republic and Slovakia, but it is common in other parts of Europe as well. Until the 1800s, Czech was known as Bohemian to native English speakers, and it is very similar to the language known today as Slovak.
Czech and Slovak are very similar languages, and most people who speak one either understand or speak the other also. However, some dialects are not mutually intelligible, and some of the more modern generation learns only one language exclusively depending on their homeplace. Since the two are so similar though, many can at least understand the basics of both languages.
Common Czech is the most common dialect spoken for the Czech language, but there are Moravian and Silesian dialects also spoken in some regions. The Bohemian dialect is spoken in a few regions, although this dialect is very similar to the Common Czech dialect. In the United States, Texas Czech is spoken by almost 15,000 people currently, although fewer people each year speak this dialect of the language as more turn to English and even Spanish instead.