Traps are men who've been led into crossdressing and submission. They're usually in sexual scenarios by either peer pressure or to fulfill fetishes. They're often drawn as women but with more diminutive female characteristics (genitals, breats).
The definition of futa varies from person to person. Some people say that a futa has both a penis and a vagina, some people will say penis/no testicles, I'm just going to rely on the most common definition; penis only. Their attitude may be that of a man, but their figure is clearly that of a woman. Futas are essentially just dominant women with cocks.