custom fields, modify db or plugin?

I’ve taken a look at the documentation page on Custom Fields, but I have some questions:

1) are custom fields only associated with posts?

I realize that to retrieve them, you must use special code in the loop, but I’m wondering how to associate them only with a specificate category of posts… can you get a subset of data from a custom field?

2) can you tap into custom fields elsewhere than with posts?

I’m wondering if this is a way to add additional information for a user, for example… or not. If not, does anyone know of a way to do that? For example, let’s say I wanted the gender or hometown of a person that signs up (registers) (I know, most won’t give it, but this is a special site where users *would*)…

Is there any way to create custom fields for a particular user, then query (elsewhere) for a custom field for a specific user_id?