I think for the most part in person interactions with a veterinarian would be the most valuable way to learn about the field. Most people begin my shadowing a veterinarian that works close to where they live. Gaining veterinary experience is very important and unfortunately volunteering at the human society typically will not count as vet experience (animal experience definitely, so its not a waste of your time, but if you don't have vet experience you want to focus on gaining that).
As far as online sources that might provide insight into the field, the only thing I can think of would be to read through scientific journals to get a taste of what research looks like. THis can be done through pubmed.com or javma or other sites. However without the background to understand the publications it would most likely not be very helpful and I don't think most schools would count that as gaining insight into the veterinary field.
If you're talking more about insight just for yourself and not something you're looking to put on an application, there are many vet student blogs that provide a glimpse of what it is like to be a veterinary student that are both extremely interesting and beneficial.
