For normal ASCII domain names, the letters a-z, the numbers 0-9, and one special character, the hyphen or dash "-". Note that domains cannot start or end with a hyphen. For the new tested IDNs (Internationalized Domain Names), the full range of Unicode characters are available. This allows nearly all of the languages on the planet to be represented within domain names, including Thai, Arabic, Lao, Hebrew, French, German etc.
Our advise always is for customers to stay away from special characters in domain names. It will only lead to confusion and you will spend your business life repeating the domain name and spelling it out to customers, instead of running your company.
For information regarding domains please visit our domain registrations section of the site which answers alot of these questions.