An asexual person, by default, does not feel sexually attracted to anyone. Like other sexualities, it is an orientation and exists on a spectrum of experiences. While not experiencing sexual attraction is the default, some people on the asexual spectrum do rarely or under certain conditions experience sexual attraction. We are not plants. Asexuality is an orientation and differs from celibacy in that we are not resisting something that strikes us when we see a person- we simply never (or rarely) feel that something.
Asexuality Visibility and Education Network (AVEN)To my knowledge, hydrosexuality is not a thing. Asexuality is not feeling sexual attraction toward anyone. Grey-asexuality is not feeling fully "sexual" or not for a variety of reasons, and Demisexuality is experiencing sexual attraction only after a close emotional bond has been formed. The bond is a pre-requisite but not a guarantee of sexual attraction.
Definitions according to AVEN's FAQTrick question! All of these are types of attraction. Just as we can see a pretty sunset but not want to have sex with it, we can see an attractive person, acknowledge their attractiveness, and not want to have sex with them. We can see someone and want to be their friend or even want to be in a relationship with them without experiencing a sexual attraction to them. This is true for people who are not asexual as well!
Graphic listing types of attractionSAM stands for Split Attraction Model, which provides a way to describe one's romantic attraction patterns separately from one's sexual attraction patterns. Romantic attraction uses the same prefixes as sexual attraction and is often the same prefix but not always.
AVEN has a short summary hereAnother trick question! To one degree or another, all of these characters are considered asexual. Monkey Luffy is stated to be asexual by his creator. Both Raphael and Jughead are asexual in their source material but less so on TV. (I would be remiss if I did not point out that Jughead is often considered aromantic as well.)
Tumblr post with pictures and descriptionsA platonic "crush" (where one wants to be friends/have a platonic relationship with the other person) is known as a "squish".
AVEN forum post with discussion of a squishAsexuals are believed to make up at least 1% of the population world wide. As the world has about 7 billion people, that makes for at least 70 million asexual people. Hopefully, more visibility, acceptance, and research will give us a more definite answer than this.
A nice Powerpoint introduction to asexualityFALSE! Now, a great many do, but definitely not all.
The AVEN FAQ deals with this better than I can.