A DDoS is an overflow of information packets sent to an IP address (usually by malicious PC ninjas). Most common connections won't be able to handle all of the packets, and will then be effectively blocked from maintaining a connection until the attack lets up.
Chances are, if you are getting intermittent lag or get disconnected from a match randomly and then able to reconnect, you are most likely not being DDoS'd. DDoS attacks tend to start slow with minor latency issues and then gradually get worse to a point where you are unable to access any outside resources.
A DDoS can only work if the attacker has an IP address to target, so your primary goal is to protect your IP address. IP addresses are naturally hard to obtain, but some programs, such as Team Speak (which we will be using as an example), allow your IP address to be found by contacts. This amounts to virtually every compromised IP address in the IDC/Games community.