I had the very same questions that you have.
The DNS and Bind 4th edition
book can answer all your questions.
A portion on the preface of the book sais:
You may not know much about the Domain Name System − yet − but
whenever you use the Internet, you use DNS. Every time you send
electronic mail or surf the World Wide Web, you rely on the
Domain Name System.
You see, while you, as a human being, prefer to remember the names
of computers, computers like to address each other by number.
On an internet, that number is 32 bits long, or between zero and four
billion or so. That's easy for a computer to remember, because computers
have lots of memory ideal for storing numbers,
but it isn't nearly as easy for us humans. Pick ten phone numbers
out of the phone book at random, and then try to remember them.
Not easy? Now flip to the front of the book and attach random area
codes to the phone numbers. That's about how difficult it would be
to remember ten arbitrary internet addresses.
 And, with IP version 6, it's soon to be a whopping 128 bits
long, or between zero and a decimal number with 39 digits.