Cote - it depends...
Some people are D/s (Dominant/submissive) all the time, which again means all sorts - a traditional 'husband in charge' relationship would fall on this spectrum, for example. A lot of 'vanilla' relationships have a more assertive partner, but that doesn't mean they respect each other any less.
Some people only do it sometimes in the bedroom, and even switch roles, either within one relationship or with different partners. Your standard 'blindfold and sexy toy handcuffs from Ann summers' could fall under this.
Some people are somewhere in between, or have periods of being more or less serious about it.
Then of course there are different interpretations of the whole thing - pain, bondage, submission and the rest can stand alone or together, there are symbolic things like collars and names that people do or don't use, people have different rules and protocols, etc. it's not all about sex - with people who are really into it it can hardly be about sex.
Subs can get in a trance like state during and/or after some activities, or get giggly and silly, because the rush is almost like doing drugs. It is very, very frowned upon not to look after your sub after doing things, even if they aren't your actual partner.
There are some really sweet love stories, and a HUGE part of it is cuddles and feeling utterly safe and protected, even though (and even because) the person protecting you also makes you very happy by doing, er, unusual practises. 
Ahem. Or so I've heard.