Well, it's not like you go through basic training or anything, when you join the Peace Corps or wear a uniform. All you're doing is volunteer work in another country for two years. You could do that on your own if you wanted to, but the Peace Corps is able to arrange it better for you. And, of course, you need to have skills in areas that are important to the local population, such as teaching, business, medical, or farming skills. Help local ppl start and run a business. Work in a local hospital. Teach English, etc. It's no different than volunteer work in your own country, except it's another country.
What it's like depends on what country and what you're doing there.