This may be a long post, because I am going to try to answer all questions at once. Please bear with me.
The easiest question to answer is why it wasn't on the HRC website as a branch. Technically speaking, PSYOP is not a branch. While the MOS exist on the enlisted side, there is no officer branch. It is a functional area. Officers coming in must be O-3 and above (in most cases). When they complete the PSYOP Officer Course, they continue to wear their basic branch insignia. Thus, you will see officers from all of the Combat Arms and most of the Combat Support branches in a PSYOP unit. Our personnel manager is also the Special Forces personnel manager. SO while our Career Management Field is 37 (not 18), we're managed by the same people. The reason for this is because we are part of Special Operations Command. Accordingly, and SF qualified NCO or officer who transfer into a PSYOP unit is allowed to wear his Green Beret.
Needless to say, we have no branch insignia for officers. We do have our own enilsted MOS insignia and regimental crest. Officially, we're the 4th Special Operations Regiment. Officers who have completed the POOC may wear this insignia. NCOs and enlisted who have transferred in from other MOSs have the option of continuing to wear their original regimental crest. I do not know the reason for this.
What do we do? Well, we persuade, influence, and convince foreign audiences to take a course of action favorable to the United States and its allies. We "win hearts and minds". We can do anything from leaflets and loudspeakers to pirate radio and TV stations. Basically, we're a marketing department with teeth. We can shore up a government or destabilize one. We can incite or prevent riots. We can respond to enemy propaganda. We can clear routes and prevent civilian casualties. We can go out to the market and bazaars and find out what is on the natives' minds. We essentially grew out of the HUMINT field. Prior to being designated 37Fs, our MOS was 97F, and part of the MI branch. This is why I started a thread in the MI board as well as this one. I know one recruiter who likes to say we play "Jedi Mind Tricks". That is a little simplistic, but not to far off. We're cultural experts, so we're supposed to be able to get inside their heads and know what they're going to do, when, how, why, and how to prevent, counter, or enhance it, depending on our objectives.
Anytime you see marketing or advertising, you are being PSYOPed. The differance between us and the civilian sector is the fact we go to the field. We can go do "clandestine" things. We also don't have the same truth in advertising rules as civilians. If you ever want to see an angery armor company commander, watch his face when he finds out the "platoon" of tanks he was chasing was actually a Humvee with a loudspeaker leading him into an ambush.
You can do some really cool things in the Humint field. The major differance is in the type of field work we do. Humint is mostly collection. PSYOP will have some collection aspects, but it more exploitation. Here is an example: HUMINT located Sadaam's sons. When the 101st rolled in to apprehend or kill them, two tactical PSYOP teams started the affair by broadcasting a surrender appeal over loudspeakers. The sons responded with gunfire, and we all know the rest.
As a side note, I would like to take a moment to brag that those two teams were from my company. HOOAH!!!!!
When people ask which they should choose between PSYOP or HUMINT, I am naturally biased toward PSYOP. As enlisted and NCOs, we often function many levels above our own pay grade. It is not uncommon for an E-4 to be the one briefing Ambassadors. We do tend to be NCO heavy, as there is an E-6 and an E-5/E-4 CPL for every SPC and below. This is the result of our association with SF. The result is you can make rank pretty quickly. We deploy a lot. There just aren't enough of us to go around. We also tend to do more "soldier work" than HUMINT. We're "brainiacs" who do grunt work. We will often (almost always) be tasked out to support infantry or SF units. By this, I mean actually go forward of the line with them, or sometimes even ahead of them.
Having said this, there are some really cool things to do in the HUMINT field. I am just biased. I always advise people to go PSYOP.
I hope I answered everyone's questions. If you have any more, feel free to ask. I try to log in a couple of times a week, and I'll be happy to answer any questions (within reason).