Asexual is a lack of sexual attraction. Ace is the shorthand term for asexual. Sexual attraction refers to sexual desire towards a particular person. Sexual desire is the want to have sex, which is not always triggered by something, and many aces experience this but I am not one of those people.
Romantic attraction is the same as being attracted to someone and feeling like you want to develop a relationship with this person, without the sexual attraction to go along with those feelings. The "pan" prefix means that I can experience romantic attraction towards people of all genders, including trans* people.
Hope this explains it well enough!