DDoSPedia is a glossary that focuses on network and application security terms with many DDoS-related definitions. It provides a central place for hard to find web-scattered definitions on this topic.
A UDP flood attack is a network flood and still one of the most common floods today. The attacker sends UDP packets, typically large ones, to single destination or to random ports. In most cases the attackers spoof the SRC IP which is easy to do since the UDP protocol is "connectionless" and does not have any type of handshake mechanism or session.
The main intention of a UDP flood is to saturate the Internet pipe. Another impact of this attack is on the network and security elements on the way to the target server, and most typically the firewalls. Firewalls open a state for each UDP packet and will be overwhelmed by the flood connections very fast.