Trans & Cis are just two different things. For example, I was born male, but now live as female. I would like to believe if you saw me in public, you wouldn't think anything of me and just assume I was naturally female. Anyways, since I was born male and live as female, I'm trans. Likewise, if one is born female but desires to live as, or does live as, male, they too are trans. The prefix in these instances is "trans-" and the suffix is the gender we desire to live as. Thus, I'm "transfemale" or a "transwoman." Conversely, for most people, nature gets the whole gender thing right and they are fully satisfied living as the gender nature gave them. These people are cis. So, therefore, if you're (general you, but mostly directed at B-Man for asking the question) satisfied being male and don't desire to live as or be female, you are a cis-man.
The whole trans/cis thing can be difficult to understand if you're cis. For the most part, you only have yourself as a point of reference so it can be difficult to understand how one can not identify as the gender nature ascribed them. It also doesn't help that most people could go their whole lives and not meet (or not know they've met) a transperson. The thing to remember is that just because someone is born with male parts doesn't mean they identify as male. Gender is in the brain and not in the pants.
Does it mean that you were interested in things marketed/intended "exclusively" for the opposite sex growing up? Seriously, I'd love to be enlightened on what makes a transgendered person different than a cis person.
As per this, it can differ from some trans-people to other. It's kind of a nature v. nurture thing and I've heard example of both. I know that for me I definitely gravitated towards girl things; toys, shoes, clothes, activities, etc all while my parents tried to push male toys & things unto me. Using transwomen here for simplicity, some of us at a young age may be successfully socialized to like boy toys, clothes, activities, but sadly it just means the gender issue will pop up later in life. Likewise, to build off of Smurfy's 2nd point, socialization can play a role in this too. I know I can be super femmie or rather masculine acting for a female. On the one hand, I do enjoy doing feminine things, listening to girlie pop music (love Lady Gaga), or I guess acting more stereotypically female. On the other hand, I still play pokemon, love metal music as well, and keep my nails short because I can't give up guitar (all things I kind of picked up from trying to be male). While I don't think I'd play guitar if I was born female, I do enjoy it now and can't imagine giving it up (as I'm sure is the same with other stereotypically "male" things).
For all practical terms, it meant that I was born with male parts, everyone assumed I was a guy, and I eventually came out and decided to live as female though various medicines & therapies, and maybe eventually surgeries as well. It means when you see me, hopefully, you assume I was born female in spite of the fact I wasn't.
Also, as Iosa said, tumblr is the last place you should look for an answer as many people try to shoe horn the "trans" label unto themselves. Smurfy gave a good explanation and if you have further questions, I would suggest you think more about what you're trying to ask. Not that you're being offensive, but you're being very vague and its hard to explain what being transgendered is anymore than the answers you've been given (born in the wrong body, seeking to live as the "other" sex). Anyways, if you have questions, I wouldn't mind answering them, and I'm sure other posters wouldn't mind as well (though don't hold me to this since I don't wish to speak for others)