>>129598079What
>>129598138 said has a few mistakes. The name "trap" stems from "trapping" as in tricking men, but it pretty much just means "a male who's attractive and feminine enough to pass for a female with only the help of crosdressing, acting and perhaps make-up". Psychology and sexual orientation don't factor into this. That's it, everything else is just combo'd on top of this, but isn't directly a part of it.
What he described as sissies are covered by that umbrella, but it's far more complex and wide than that. It's a psychological fetish even before being about looks, focusing on both increasing of the feminine and the erasure of the masculine, with humiliation and submission playing important parts. It's also not about becoming a woman and shouldn't be confused for that. The femininity it features is often exaggerated in a purposely garish and silly way. Neither is it about the sissy being gay, although it can heavily feature gay activities.
Basically, although these two may sometimes look similar, they are quite different in the psychological and physical aspects and also in what they entail for the practitioner. Although obviously they can overlap or transition into each other if the practitioner has a fetish for both or it evolves.