Shield is an OVH-developed software solution that runs on vRouters. Its purpose is to mitigate known attacks, mainly those that work via amplification (DNS Amp, NTP Amp...)
Armor is the most advanced VAC software solution, designed to mitigate advanced persistent attacks. It runs on vRouters with FPGA cards in order to reduce the CPU load on part of the processing, and obtain the best performance levels on complex algorithms.
Mitigation is a term employed to design the means and measures in place to reduce the negative effects of a DDoS attack. Mitigation consists of filtering illegitmate traffic and hoovering it up with the VAC, while letting legitmate packets go through.
The firewall network is composed of vRouters executing OVH-developed code, enabling all traffic to be classified so that rules can be applied (access-lists).