One of the first questions I'm asked by those who are familiar with Foreign Service Officers is, "What's your cone?" or "What track are you?" There are currently 5 different career tracks for Foreign Service Officers (FSOs, also known as "Generalists"), so this post doesn't apply to the many, many other roles that fall under the "Foreign Service" umbrella.
The 5 FSO career tracks (also known as "cones") are: consular, economic, management, political, and public diplomacy. You choose one when you begin the FSO application process.
So what's the point, especially if everyone applying is being judged based on the same criteria? Some will tell you that certain cones are "easier" for getting into the Foreign Service than others. Some of those same people may suggest to you (as they did to me) that "you can always change to the cone you really want later." I don't recommend this strategy. From what I've heard, it's quite difficult to change cones. You should go in assuming that whatever track you chose when you applied will be your primary track for the rest of your Foreign Service career. Moreover, people from all cones are being selected in each class of new diplomats. One of those lucky applicants could be you! After all, choosing the track that best resonates with your background, skills, and personality should help your candidacy. tl;dr Pick the cone that you actually want.
Which track are you? I'm Public Diplomacy, also known as "PD." My cone includes roles in educational and cultural diplomacy as well as press and media diplomacy. PD work can range from promoting studying in the U.S. to international students to being grilled on U.S. policy on foreign language television.
So can you only do Public Diplomacy jobs? No, tracks aren't like that. Many FSOs do tours outside of their career track during their careers. The cone still indicates each FSO's primary track, where he or she will serve most frequently, but jobs aren't so restricted that all other opportunities are closed. In fact, one of almost* everyone's first two tours must be consular, regardless of his or her cone.
If you want to learn more about the 5 FSO career tracks, I highly recommend starting on State's website here. If you would rather talk to a human being, Diplomats in Residence (DIRs) are a wonderful resource, too.
*I've heard of waivers being granted for exceptional circumstances, but they seem to be extremely rare.