There are several types of DDoS attacks. The best known forms are volumetric and network protocol attacks. Volumetric attacks stuff incoming connections to a targeted site with more bits than the connection can carry. This is akin to routing more vehicles onto a highway than it can accommodate. Eventually, the traffic comes to a standstill. As of last year, the biggest recorded volumetric DDoS was 3.47 terabits per second.
Network protocol DDoSes work to overwhelm routers and other devices found in layers 3 and 4 of the network stack. Because they work on these network layers they’re measured in packets per second. One of the largest protocol attacks was one blocked by security firm Imperva that peaked at 500 million packets per second.