If you take a look at my map of the most common surnames in Europe, you will see that Nowak, Novák, and Novak are the most common surnames in Poland, Czech Republic, and Slovenia, respectively. But what do they mean?
Nov- or Now- is a Slavic (and not only Slavic) root meaning “new” (you may notice that it looks similar to English “new” or Spanish “nuevo”, and they are indeed derived from the same root), and -ak or -ák is a Slavic suffix that makes another word into a person (for example, Pol- is a root for something Polish, and Polak or Polák means “a Pole” in many Slavic languages).
There are several different reasons why a person could be called “new”. The most common one is that he moved to a new area, so the people around him called him literally “new man”. Note that I write “he”, not “he or she”, because surnames in Slavic languages usually have different variants for women. The wife (or daughter) of someone called Novák would be Nováková, literally “new man’s”.
Another, less common reason is that the person could be new in his profession or craft. For example, when there was an old smith, called simply Smith, a new young smith could be called Nowak, Novak, etc., to distinguish him from the older and more established one.
Finally, let’s take a look at different spelling variants from different countries:
Novák – Czech Republic, Slovakia
Novak – Slovenia, Croatia
Новак – Russia, Ukraine, Belarus
Furthermore, even though Novak is not a common surname in Serbia, Bosnia, and Montenegro, it is used there as a given name (you have probably heard of Novak Djokovic, a Serbian tennis player). The (patronymic) surname derived from Novak in these countries is Novaković (-ović is a Serbo-Croatian suffix referring to the son of someone; that is, Novaković means “Son of Novak”).