Because US society places a lot of value on independence and personal freedom, the stereotypical reaction to the concept of submitting to another person is often one of revulsion; because it is a kink, it is almost never talked about casually (and when it is, it is often given labels like 'sick' or 'perverted'): these two things combine to make discovering and dealing with one's submissive tastes a rather difficult and sometimes traumatizing task.
Where do these submissive tendencies come from, and what exactly are they? Like all kinks, submission is a matter of taste. Who can say exactly why I happen to like lime sherbert, except from some unknown mixture of genes and experience, nature and nurture? The same has been said of one's sexual preference. Very little research has been done, no doubt because of the taboo nature of non-vanilla sex.
You may think I'm being redundant when I say that being submissive isn't sick, wrong, twisted, a perversion of nature or indicative of weakness, but it needs to be said. A lot of people know this but don't necessarily feel it; you cannot spend the rest of your life knowing that you're submissive and hating yourself for it.
The strength and scope of these tendencies varies widely. To start with, they can be broken down into two broad categories: sexual and non-sexual. Concerning the non-sexual 'everyday' behaviors: some people like to lead, some are only happy when they serve. People at the extreme ends of this spectrum can exhibit behaviors that most people consider negative: extreme bossiness and rudeness (think overblown type A personality) or indecisiveness and an inability to stand up for onesself. Psychologists may use the terms 'submissive' and 'dominant' to describe such behaviors, and the reader should be aware that in this context they probably aren't referring to sexual D/s.
There is no clear relation between these personality traits and one's sexual tendancies - the everyday personalities of submissives span the entire length of the behavioral spectrum mentioned above.
Thus, in normal life, submissives are not necessarily pushovers - in the world of D/s, it is often heard that subs are some of the strongest people out there, i.e., strong of mind and heart. How you react to the general populace is going to be different from how you react to bedroom partners, and Dom/mes and subs are no different.
The sexual tendancies vary in strength - some people just like their partner to control a session of what would otherwise be vanilla sex or like to role-play at serious D/s; at the other end are people who are highly aroused by thoughts of being sexually dominated, and have a strong desire to experience such. The scope can range from pure D/s to including some of the more commonly associated kinks like sadomasochism (S&M) and bondage, to less-common kinks like knife play and asphyxiation.
With some people, D/s is a need... they might not be able to have a successful romantic relationship without it. With others, it is a strong desire, but they could maintain relationships without it; with yet others, it is a little extra spice that is thrown in here and there. For the latter, the dominance and submission could almost be said to be role-play; for the former two it is a more serious thing.
The concept of the sub (submissive) actually being in control of an encounter (a 'scene') is known as 'topping from the bottom' and is, I will argue, not necessarily true, especially in the serious D/s relationships. By the very nature of D/s, it is the sub's limits that are being pushed for the most part, and they are the final authority on when they've been pushed too far; any Dom/me who does not carefully consider those limits is going to have trouble keeping partners, as has been mentioned.
However, every time the sub does obey the Dom/me, they (the sub) are being controlled. Every time they let the Dom/me push their limits, they are relinquishing a bit of control. In a serious relationship, the sub is not getting every wish fulfilled at their beck and call; any sub who would engage in wheedling, whining, barganing, or constant manipulation is considered to be topping from the bottom; such behavior is generally destructive to the relationship. After all, being controlled is at the core of what most, if not all, subs are looking for in a D/s relationship. It can be a powerful thing to let yourself be made helpless at the hands of another, to trust them utterly to take care of your mental and physical self.
The Dom/me also has needs that the sub meets - in this respect a D/s relationship is just like any other relationship, or even a close friendship. Both people have needs that the other can meet, but those needs aren't going to be exactly complimentary, so each person is (hopefully) going to do some things that they don't necessarily like to do just because they like making the other person happy. This tends to be a visible feature of a serious sub's behavior, to want do everything they can to make their Dom/me happy.
You don't necessarily have to be a Gorean to operate without safewords, or (as I have already pointed out) to have the Dom/me truly in control. Goreans are a particular variant of the total power exchange, but not by any means the only one.
A common theme of submissives is that through submission, they can cast off the cares of normal life for a time, drop all their barriers and their armor and just be, knowing that their Dom/me is protecting them, looking out for them, keeping them safe... and in a sense, they are looking out for their Dom/me even as they submit.
Books can and have been written on the subject, and I encourage further reading. The web is a blessing and a curse, for while it provides a (usually) safe and (usually) private way to learn about D/s, the material that is out there tends to be produced by the people at the far ends of the spectrum. There is good material; you just have to dig for it... and for a person with no prior experience and no one to talk to face-to-face, telling the good from the bad is not easy, and should probably have a node made for it at some point.
Random bits of terminology: a person who can be (and I'm not talking about just playing the role) both a Dom/me and a sub is called a switch. Top and bottom are often used synonomously with Dominant and submissive, respectively, but some people do recognize subtle differences between the two sets; the former being weighted more towards S&M activities (which do include a type of power exchange,), and the latter more towards activities that have dominance and submission at their core. The term slave in its loosest sense denotes a sub in a serious D/s relationship and is synonymous with being collared.