RDAP is the Registration Data Access Protocol developed by the IETF as the successor to whois.
Key difference:
whois returns free text, RDAP returns JSON, making it machine readable and easier to automate. It also supports RESTful web services, allowing for HTTP based queries, error codes, authentication and access control.
RDAP also supports Internationalized Domain Names (IDNs), which are domain names utilizing non-Latin characters. Languages can include Arabic, Chinese, Cyrillic or Devanagari. As DNS is limited to ASCII characters, an ASCII encoding called Punycode (deliberately designed to rhyme with Unicode) is used for name translation.
All that said, whois is still probably more frequently used than RDAP.
