A SYN attack is a type of denial of service attack in which an attacker sends SYN requests to a targets system in attempt to consume enough server resources. A SYN attack is also known as a half open attack. Majority of SYN flood attacks can be blocked via Firewall rules.
A DDoS attacks goal is to saturate the bandwidth of the attacked site with malicious traffic, which will require other means of filtration. However there are several different types of attacks and not all are high volume attacks.