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Domain names can only contain letters (A-Z), numbers (0-9) and hypens (-)

The exceptions to this are IDNs (internationalized domain names) that can contain non-ASCII characters..
The IDNs doesn't really contain non-ASCII characters, but ASCII (letters/numbers/hyphens) are used as "codes" for non-ASCII characters..
you can read more about IDNs at Wikipedia here:
http://en.wikipedia.org/wiki/Interna...ed_domain_name

There are several hundred domain extensions available, but the most well known are the Generic Top-Level Domains that are open for registration for everyone;
.com : For "commercial"
.net : short for "network"
.biz : Business
.info : Information
.org : Organization

There are also several other restricted top-level domains like:
.mil : Restricted to the US Military
.gov : US Government
.edu : Educational institutions

Here's a complete list over generic top-level domains:
http://www.iana.org/gtld/gtld.htm

And some info on each of the extensions on Wikipedia:
http://en.wikipedia.org/wiki/List_of...-level_domains


The other "type" of domain name extensions are country code top-level domains (ccTLDs)
These are generally used or reserved for a country or a dependent territory, and many of them are restricted to residents of the country.
Some examples:
.us : USA
.ca : Canada
.ru : Russia
.se : Sweden
.no : Norway
.de : Germany

Here's a complete list of ccTLDs:
http://www.iana.org/root-whois/index.html

And some more info on them from Wikipedia:
http://en.wikipedia.org/wiki/Cctld

Hope this answers your questions..