There are two types of IDN, top level and second level.
Top Level IDNs
Top Level (or full) IDNs have the same script at both the TLD level and the domain label.
Second Level IDNs
Second level IDNs have a non-ASCII label under an ASCII TLD.
Second level IDNs have been available since 2000, both in the ccTLD and gTLD spaces. Latin script IDNs (eg with accents and diacritics) have a natural affinity with ASCII TLDs such as .com or .eu.
It was not possible to register full ccTLD IDNs until 2009, when IDN strings were allowed into the root IANA database through the ccTLD IDN Fast Track process.
At the gTLD level, gTLD operators had to wait for ICANN to open the call for a new gTLD round to be empowered to submit applications for IDN TLDs, which happened only three years ago, hence the low percentage (1%) of top level IDNs in the gTLD space.
In December 2014, 72% of registered IDNs were at the second level, and 28% were full IDNs. However, there has been an expansion in IDN top level domain registrations since their introduction in 2009.
As at December 2015, there are 2.2 million top level IDNs and 4.5 million second level IDNs.