From securityonline.info
Syborg is a Recursive DNS Domain Enumerator which is neither active nor completely passive. This tool simply constructs a domain name and queries it with a specified DNS Server.
When you run subdomain enumeration with some of the tools, most of them passively query public records like virustotal, crtsh or censys. This enumeration technique is really fast and helps to find out a lot of domains in much less time.