The difference when you use them as verbs describing human behavior depends on the kind of animal they are used for. Herding is used for herd animals. If humans are said to be herding onto a sidewalk, they are to be pictured moving relatively slowly and in a somewhat orderly fashion. Flocking is commonly used for birds, so when used for humans, the verb "flocking" would imply more bird-like behavior. It would be less orderly, and more quickly than herding. Swarming is used for insects. If humans are swarming, then they are moving very quickly, probably pushing to get in a better position. Usually it is used in the phrase "swarming around [something]." Probably because humans rarely exhibit swarm behavior unless there are many people frantically or chaotically trying to get to the same place. This only applies to verbs. A school of fish doesn't school around, so you wouldn't say that humans are schooling, without it explicitly being a metaphor.

There is no real difference. It is simply custom. We always talk about a herd of cattle and a flock of sheep. There is no real reason that it could not be a flock of cattle, but it would simply sound a little strange. We also talk about a colony of ants and a troop of baboons. It's simply the way the language developed. If you want a strange one, a group of owls is called a parliament of owls. Try this site for more: http://www.thealmightyguru.com/Pointless/AnimalGroups.html

Thousands of Green Line passengers herded onto sidewalk to wait for shuttle buses in a subzero-degreee windchill. The difference when you use them as verbs describing human behavior depends on the kind of animal they are used for. Herding is used for herd animals. If humans are said to be herding onto a sidewalk, they are to be pictured moving relatively slowly and in a somewhat orderly fashion. Flocking is commonly used for birds, so when used for humans, the verb "flocking" would imply more bird-like behavior. It would be less orderly, and more quickly than herding. Swarming is used for insects. If humans are swarming, then they are moving very quickly, probably pushing to get in a better position. Usually it is used in the phrase "swarming around [something]." Probably because humans rarely exhibit swarm behavior unless there are many people frantically or chaotically trying to get to the same place. This only applies to verbs. A school of fish doesn't school around, so you wouldn't say that humans are schooling, without it explicitly being a metaphor.

I think the word flock is only associated with sheep, and that probably has something to do with how their behaviour in a group (herd?) is noticeably different from groups (herds) of other animals. They are more easily led, or influenced, less independent. So that probably puts them in a different category in people's minds than other animals that are more independent, but still capable of being driven as a herd (horses, cattle, people?). There is an expression "herding cats" which is supposed to bring to mind something impossible, because they are so independent. They are not herd animals at all.