I think the difference between a cross-dresser and a trans woman would be that cross-dressers have no cross-gender identity. Despite wearing the clothes and adornments of the opposite sex, they still identify as their birth sex, and have no problems with that either socially or bodily. But they like to express themselves in the manner of the opposite sex. A male cross-dresser would probably see their cross-dressing more as "expressing their feminine side," in the manner that it purely is presentational self-expression, but without actually wanting to be a woman physically or socially. He would be content living his life as a man who expresses himself in feminine ways.
A transsexual person identifies as the opposite sex irregardless of clothes and adornments. What's important to them is either having the body of the opposite sex, or being socially accepted as a member of the opposite sex. They want to be treated as the opposite sex always, and their fundamental gender identity, how they see themselves, is that of the opposite sex. A crossdresser presumably identifies as their birth sex, and yet enjoys dressing in ways associated with the other, even though their internal sense of their own gender identity is not actually that of the opposite sex.
Also, presumably, and this is my own opinion, as we're coming toward more cultural acceptance of male-bodied people expressing themselves in feminine ways, I expect that some crossdressers very well might become like the male equivalent of a masculine lesbian... someone who wants to wear clothes that are culturally "supposed" to be for the opposite sex, look beautiful instead of handsome, be able to wear "feminine" things, have longer hair and wear jewelry and the like, and yet still socially be seen as male. Because they're presumably more interested in the feminine things themselves, and in being beautiful, and in being able to be appreciated for that gender expression, than they are about actually being accepted as female by other people.
Genderfluid would be more that your internal sense of which gender you are and which gender you want people to see you as and accept you as changes... it moves between several different identities and desires acceptance as those identities. A genderfluid person would hypothetically dress as a woman because they actually felt like a woman that day, not just because their same unchanging identity just decided being pretty and feminine.
It's a confusing categorization thing, since technically "cross-dresser" is also an umbrella term for someone who wears the clothes and adornments typically associated with the opposite sex, so it really isn't a special separate category that separates it from someone with a transgender identity. So feel free to correct me if I'm wrong, anybody... I'm just basing this on my limited experience with the one or two self-identified cross-dressers who come to our local trans* support group.
