I feel as though I've been living a simple life for quite a while now, but I can't say that I'm completely frugal. I believe that mamabear is right that they do overlap, but can also be very seperate. For our family, the simple life has always been about leaving the smallest footprint on the earth as possible. I enjoy looking next door at the neighbors and realizing that we have half as much "stuff", and we're twice as happy.
Our lifestyle choice has always been about living intentionally, that EVERY decision we make impacts all aspects of our life on earth. There are a lot of things that I spend money on that I'm sure other people would think it insane. Everything we eat is either organic or grown by someone we know. We spend a lot of money on food, but I believe in the manner the food was grown and in the quality of the food. I do not buy name brand (Gap, Gymboree, Nike,...)anything because I do not wish to support their business practices.
{stepping off the soapbox now
}
If you have a chance I really recommend the books the other mama's mentioned, just add to the list "Fast Food Nation". It's a really good book that will make you question a lot.
It really is about choices and intentions, the fact that you are questioning and examining your lifestyle puts you so far ahead of others that just buy out of habit.
Good Luck
Kaight