DDoS stands for Distributed Denial of Service, a malicious attempt to make a server or a network resource unavailable to legitimate users, by overloading it with massive amounts of fake traffic.
Historically, DDoS attacks are associated with hacker and hacktivist groups and often considered to be a work of professional cyber crooks.
In recent years, with the emergence of DDoS-for-hire services (a.k.a. stressers or booters), the barriers to entry for a DDoS attacker has been significantly lowered, offering users the option to anonymously attack any target, for just a few dozen dollars. (source)
In a nutshell, what these DDoS services are usually selling is access to DDoS botnets: networks of malware-infected computers, which are in turn being “subleased” to subscribers.
Sadly, building such a botnet is simpler than you may think, considering the elaborate damage that can be caused a cluster of such “zombie” computers.