To elaborate on the existence of ů (u with ring above): The
rule of thumb is that it appears
if and only if you hear long 'u' inside native
Czech words, which is true, but it's more or less coincidential. (Note that this doesn't apply to
loan words: for example "kůra" means "tree bark", whereas "kúra" means "treatment procedure". They are both pronounced the same, kinda like 'koo-ra' with a rolling 'r'.)
The point is that in old
Slavic languages, there was a long 'o' (ó) in place of the current ů. This has since differentiated in a number of ways in each of the languages - in Czech and
Slovak it turned into "uo". While Slovak has remained in this stage (the
diphthong is now spelled 'ô'), Czech (especially
Bohemian dialects) went on to long 'u'. I've heard somewhere that the ring above ů is to resemble the original 'ó' sound, and for all I know, it may be true.
Anyway, the existence of ů has
ethymological reasons.
And by the way, Balto-Slavic family of languages? Sub-family, perhaps. "Family" is a higher rank, and Slavic languages belong to the
Indo-European family of languages.
Sorry if I ramble too much, I just like to discuss stuff like this. :)