A professor of mine once described public health this way: Two young men were fishing in a river. As the older of the two men cast his bait out to the middle he saw a body floating down the river before him. He was awestruck as another body floated by, and then another one. When his partner began to run from the river, the older man asked "Won't you help me pull these bodies out of the river?" to which the young man replied "I'm going to see who's pushing them in!"
When I say I work in Public Health the first question I get asked is "Are you a nurse?" Nope.
"Oh, do you work in a lab?" Nope.
I don't fault them for this--the field as a whole is still vying for its niche. Basically, what we do can range from coordinating social research projects (like what I do) and conducting research, writing reports, etc. to becoming an advocate for tobacco prevention (for example), to putting up mosquito nets to ward off Malaria. Public health plays an important role in disease prevention in developing and developed countries.
The key component of public health is that it is focused on a) prevention rather than the cure (or an upstream approach), and b) it deals with population-level, rather than individually-focused health issues. In other words, we are interested in improving the health of the community as a whole as opposed to an individual intervention.
Be safe and healthy!