Okay, so DDoS stands for Distributed Denial of Service. Distributed alone indicates that for an attack to qualify as a DDoS, the attack must be distributed across multiple machines (meaning that there must be more than one attacker).

If you were to simply DoS your localhost, you probably wouldn't be able to do much (I doubt DoSing anything would do very much), unless you used something like a Slowloris and you were running Apache.