The History:
Heraldry, as we use the word in the SCA can best be traced back to medieval England, specifically the 12th century AD. These court officers were more than just human PA systems, they were required to he literate to some extent, as well as fluent in the nomenclature and laws regarding their profession.
Keep in mind that the historical definition of herald becomes less and less well defined the farther back in history you go. Following the fall of Rome, the governments of Europe were forced to evolve out of comparative chaos, meaning that many of the roles we traditionally attribute to heralds today may not have even existed, or been formally recognized as 'jobs' yet.
The Society:
Well, make no mistake about it, heraldry as a subject had firmly re-evolved within the SCA as a massive bureaucratic force that handles many administrative jobs across the kingdom, ranging from list heraldry to registering and recording devices to Calling courts for all levels of nobility and royalty.
SCA heraldry as it is practiced today is a wide ranging set of disciplines that can be broken down into three overarching categories.
1) Voice heraldry: which generally includes any and all who are required to project their voices, this includes court heralds, List heralds and site heralds.
2) Book heraldry: this includes anyone who researches the accuracy and appropriateness of names, coats of arms and badges, as well as screens new applicants for the same criteria. Books heralds also help to record registered names and devices to prevent conflicts within the society.
3) Administrative heralds: While this is not generally broken down as a third and separate category, part of the structure of the modern day college of heralds includes its own bureaucracy to help move the flow of forms and and applications up and down the administrative chain of command. While these people my never help someone file a submission for name, and may never herald a court or a list in their lives, they are still heralds, and many of them work just as hard or harder than most people give them credit for.
The Reality:
Well, somewhere between history and the paperwork lies reality. The truth of the mater here is that 'heraldry' has become a term that encompasses a large number of disciplines within the society. Not only are heralds warranted officers in some cases, but they can also be freelance independents, deputies, regional or kingdom officers, or just someone who has another more prominent role in the SCA who just happens to also be able to do heraldry.
Most heralds can not and don't neatly divide their jobs up along the categories I pointed out in the previous section. The truth of the matter is that the local herald to a group usually has to have a solid grasp on, but not be expert in all three of the roles I outlined, and usually they will have deputies to help them where their skills are lacking.
Others still do not serve as formal heralds, but make use of their vocal talents and offer to help herald the site at events, or the list field at tournaments.
Personally, I consider these people to be the grunt infantry of the SCA heralds, as they do a physically demanding job with little recognition and no glory, and yet they usually do it gladly and with no complaints.
The Final Word: SCA heraldry is, for the most part, what you get out of it. While it is often divided up into is respective fields, each separate area of interest can be studied or preformed independently without any requirements to branch out into other fields that you may not have any interest in. While their is always an encouragement to learn more and expand your knowledge base, as their is (and should be) with any discipline, you wont be fired or dismissed from work as a herald if you are good a site heraldry, but don't want to herald court, for example.