The aviation industry is chock-full of acronyms. If the average instrument-rated pilot reads that you called FSS to check the NOTAMs at KPOC and the LLZ was OTS on the ILS 26L so they were using the VOR-A approach, which restricted you to a MDA of 1800', which is 789' HAA, because your DME is INOP...they'd understand what you're talking about! What we all need to remember, then, is that those outside aviation have little or no exposure to the jargon, and simplify & explain lest we bore them all to death.
The short answer, Jennifer, is that "FO" stands for "First Officer," or what many people outside of aviation call the "co-pilot."
The term is nautical in origin, as are many things in aviation. The First Officer, otherwise known as the First Mate, was second-in-command, just below a ship's Captain, and was responsible for seeing that his orders were carried out. In aviation, the first officer is designated as second-in-command while the captain bears the authority and responsibility delegated to the pilot-in-command.
A common public misperception is that "co-pilots" are "pilot trainees," who've very recently learned to fly and must build the necessary time before they can be qualified as "pilots." While this may be true in a few very limited cases (here's looking at you, Gulfstream Int'l right-seaters!), usually an airline first officer has extensive experience as captain or pilot-in-command at a previous job. Because all airliners require both a captain and a first officer, newhire pilot at any airline start as first officers, and become captains once they are about half-way up that airline's seniority list. If they switch airlines, they start over as first officers.
A common quote in aviation is that airlines hire captains, not first officers. In other words, they want somebody with the experience, knowledge, and skill to be a competent captain, even though they may serve several years as a first officer before upgrading. After all, things change fast in the aviation world, and when airlines expand rapidly, you'll find first officers becoming captains rather soon after being hired (not a concern at my company, I'm afraid!).
So what makes my job different from the Captain's? Well, I'm paid about a third of what he (or she) is. Only they can taxi the airplane, since the nosewheel tiller is on their side of the cockpit (the left side). They're expected to make the tough decisions when in tight situations, and they take responsibility for those decisions - although, should something go wrong, the FO will surely share the blame. Other than that, though, it's pretty much the same job. On each flight, one pilot is designated "Pilot Flying" and the other is "Pilot Not Flying." The "PF" simply flies the airplane while the "PNF" take care of radio communications, navigation, and aircraft systems management. Usually, the captain will be PF on the first leg, I'll be PF on the second leg, and so on.
Hope this clarifies it for you, Jennifer. Any reader, always feel free to ask for clarification when the jargon gets to be a bit much.