Reverse DNS Lookup tools resolve a given IP to Host. The resolved host is the public host behind that IP. This tool is also called IP to Hostname or Reverse DNS Tool.
Reverse DNS Check tool queries the given IP to resolve to a hostname. The hostname is anything like a normal domain or sub-domain, i.e. google-public-dns-a.google.com. This hostname is Google's hostname against IP 184.108.40.206 which is Google's public DNS ip. Whereas if you enter your ip, it might point to the hostname of your ISP, or if you query your server's IP, it might show you your domain name on which it resolves. This tool is named "Reverse DNS" just because it is a reverse process of resolving the hostname to IP just like a regular DNS check.