An Internationalised Domain Name is a domain name that can contain characters not defined by the ASCII standard. These characters include most of the accented letters used in different European languages and other characters that are not found in the Latin alphabet. (i.e accentués.com)
Technically, accented characters can not be used in DNS, which is why every IDN is encoded as a string of non-accented character whose shape is called ACE. In the latter, the domain names begin with XN - to differentiate unaccented domain names. (i.e : xn--accentus-h1a.com)
IDN / ACE converter
This tool can be used to convert an international domain name into the corresponding ACE chain and vice-versa.