Ema es bonita = siempre es bonita....Bonita siempre Ema está bonita = En este momento...bonita ahora

When you say "es" is always, no matter what. When you say "estar" it means she's beautiful for now. Maybe you're watching a TV program and you say "está está bonita" you mean for now, because she's wearing nice clothes and makeup. You can't really tell if she's always that beautiful. Also when you say "ella es bonita" it has a nuance of her essence. You mean she's beautiful inside and outside, she's kind and lovely. This is not a rule, but it often happens. (:

I agree with what Juan Carlos just said. I'd like to add that in Spain we use a lot more "guapo/guapa" when referring to a good looking person, and "bonito/bonita" more to referring to objects or little girls/boys. Ella es muy guapa Ella está muy guapa hoy

Buenas, La diferencia radica en que cuando usas "es" te refieres a atributos como buena, amable, sincera, que son comunes en esa persona. En el caso de está, te refieres a una situación más concreta.