As others have mentioned, “shemale” is a term for a certain type of fetish pornography, roughly equivalent to “dickgirl” or, for you hentai enthusiasts, Japanese futanari. It is very offensive to call an actual trans woman any of these names. A good rule of thumb is that if it autocompletes in PornHub’s search engine, you should check with a real trans person before saying it.
A trans woman is a person who identifies herself as a woman, but was not designated female at birth, sometimes abbreviated as DFAB. This usually implies being designated male at birth (DMAB), but intersex people are as likely as anyone else to identify as a woman.
As “shemales” are porn actresses, what they have in their pants is a crucial part of the job. As trans women are people, what they have in their pants is, in technical terms, None Of Our Business.
tl;dr “shemale” is a fetish porn genre, trans women are people.
As always, I invite actual trans people to tell me if anything I say on the topic needs correcting. While I have several trans friends, this does not qualify me to speak on their behalf.