A horde of zombies assaulting a school bus!
A zombie image from the movie I am Legend staring Will Smith

Before we read about what classifies a herd, horde, or swarm, I will state the obvious. If you are in a city or highly populated area, it is very likely to find a horde. If you do find a horde, take important belongings and travel to a low populated area. Hordes have the capability to take over large military compounds. A Herd, also known as a Horde or a Swarm, is a large group of zombies. They are usually attracted by sound, leading a few individuals to come together, eventually finding and merging with new groups, growing larger and larger and even more ferocious. Herds wander around without a real purpose until attracted by sounds or prey. Herds are notably more aggressive than smaller groups, and the presence of zombies around other zombies almost creates a "synapse", or "hive-mind" effect, with them able to react more quickly by watching what others do rather than acting on their own. Herds can smash through the heaviest of barricades, tear down fences, and completely overwhelm humans in waves of undead bodies, and are the zombie population at its most dangerous.

"Zombies in a herd are a force of nature. They don't operate on logic or reason. If one of them even so much as brushes a hand against your door [where you are hiding]--and another one sees that, mistakes that as an attempt to get in--it's over. That one starts trying to get in--the one who did the accidental tap thinks something's inside all of a sudden--he starts beating on the door with him. They would kill you all." - Eugene Porter