The diference is: A DoS (Denial of Service) is a single machine attack, and DDoS (Distributed Denial of Service) is a multi machine attack that can use botnets. An DoS attack can have multiple purposes, so for a good tutorial google it
As far as i know in Japan is not prohibited to you buy these tools btw using it can be privacy invasion or anything else... Is best to you search about Japanese cybernetic laws