There is a committee that decides what the TLDs (top level domains) are going to be. For example: .com .net .biz, etc. They will then announce when registrars like godaddy.com and register.com can apply to sell a TLD on the public market. Once a company like godaddy.com is "okayed" to sell Domains within that TLD, they will sell and constantly refer back to the InterNIC and ICANN (Same company more or less) sites for availability concerns.
IANA though is the top level organization here. They pay attention a keep control over what happens to all top level domains. IANA was independent from ICANN a while back, but ICANN owns IANA now. So, ICANN pretty much runs the show. But, to say that anyone "owns" the .com or .net TLDs is tough. People take ownership and are assigned to take control over what happens to them just to make some sort of order. But, essentially, that's almost like asking who owns Earth? Since our DoD did create the internet, we have a lot more control over what happens to it....hence these companies control.