What is the Foreign Service?
There are two main inroads to the foreign service being the State Department and US Agency for International Development (USAID). The recruitment process has some similarities but also many differences. Also, the type of candidate background each one is looking for is also pretty different. I work for USAID.
For those who are interested Wikipedia has a pretty fleshed out article on the foreign service going into the history of its creation and much information about it.
http://en.wikipedia.org/wiki/US_Foreign_Service
Also, for those who might want to really understand the foreign service and the job, I highly recommend the book "Career Diplomacy". Especially if you might consider a career as a foreign service officer (FSO), then this book will be a great resource.
http://www.careerdiplomacy.com/
What is USAID?
USAID is the primary US government Agency that administers US foreign assistance. USAID was formally established November 3, 1961 by President Kennedy; however, US foreign assistance has its origins with the Marshall Plan following World War II.US foreign assistance programs consists of less than 1% of the federal budget and has operations in all around the developing world. USAID locations are typically co-located with the US embassy in a country or in separate buildings nearby.
The mission of the organization is to
We partner to end extreme poverty and to promote resilient, democratic societies while advancing our security and prosperity.The general philosophy behind foreign assistance is that through promoting the stabilization of conflict areas or fragile states, then those nations can have the opportunity to grow and develop into partners and allies for the US. This will open up new trading opportunities for the US (ex. South Korea, Taiwan, and maybe soon much of SE Asia) as well build political ties to support US security (ex. alliances, partners to assist in the security of the US, and stable secure nations are less likely to promote international terrorism).
Additionally, there is a moral imperative in which the US desires to save lives and assist in alleviating global poverty, health issues, and humanitarian crises. This imperative is a responsibility of being one of the world's super-powers but also reflects the values and ideals of the American people to the world abroad.
Learn more at http://www.usaid.gov/who-we-are/ and at http://en.wikipedia.org/wiki/USAID.