Trans - what IS that, really?

Why is it that if someone says they are 'trans' and claims I should respect that, they don't tell me what they actually mean? Is a trans woman a woman 'transed' to seem like a man or is it a man who seems like a woman? Are they men/women with anomalous or surgically altered genitals; are they psychologically deranged guys/gals who simply think they aren't, or are they just people playing silly buggers? If they can't be explicit about their condition why don't they STFU about it?