Pansexuality: sexual attraction to people of all gender identities and biological sexes.
Biological Sex: merely refers to how the person's body is structured. If you have a penis, surprise, you're a male.
Gender Identity: a person's perception of their gender. Gender identity corresponds to the norms society has given to males and females. So, for instance, a person can be biologically female yet "feel like" or identify as a boy. The person really only has to say that they identify as a boy to fit the mold, but the feeling displays itself in the person's preference for clothing atypical of girls and/or refusal to shave body hair, among other things. This is a pretty crude definition so don't take it at face value.
A pansexual is just anyone with the potential to be attracted to both sexes and any of the many (and I mean it when I say there are a bunch of them) gender identities.
The primary difference that seperates bisexuals and pansexuals is that the former is only attracted to males and females who identify as their respective genders. A true bisexual cannot and will not feel attracted to a transgendered person, i.e., someone who is biologically male yet identifies as a woman (you know, drag queens). As for transsexual males-females who underwent an operation to physically become males and almost always identify as such-I'm not sure. Seeing as how they're basically just males, an unaware bisexual wouldn't know the difference. :P
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Romantic Love: purely relates to the emotional side of a relationship.
You can feel a deep connection to someone else without wanting to have sex with them. An asexual heteroromantic person, for example, desires that sort of connection but is by and large unstimulated by what others would call sexual activity.
Some people feel the need to "explicitly specify" this (as you put it) because they really have no interest in sex and it's important for their partner to understand that.
Hope this helps.