The way I understood it, Asexuality is like a massive spectrum and although it varies in certain things from person to person, the core of it is - they aren't sexually attracted to anyone. Like, if you're a straight guy, imagine the way you feel towards other guys - you're just kind of 'meh' about it, it doesn't really 'tickle' anything inside of you that makes you go 'mmm yeah baby'.

The lack of 'mmm yeah baby' towards everyone is the thing Aces have in common, everything else can vary - some still get sexually aroused and enjoy sex, some don't but enjoy the intimacy and making their partner happy, some never get aroused at all and dislike having sex - it depends on the person, is how I understand it.