How does Google determine if it is relevant links?

[Xiaoliao]

When another website links to your website, how does Google determine if this link is relevant?
For example, my site is about magazine subscriptions, there are also many magazine categories pages, now, I want to add a link to my friend's site, it is a site about action figure collection, there are two category pages where I can put the link on:
1. about hobbies and games magazines page.
2. about general interesting magazines page.
So, which page is more relevant to my friend's site?
Would the two links pass the same PR value to the action figure collection site?
Everybody said that google only calculate those relevant links PR value to a site, if a link comes from an unrelated site, this link is not a good link to your site, is it right?
So, how does google determine if the link is relevant?
Is adult and sex relevant?
Is magazine and book relevant?
Is interesting and hobby relevant?
All these are subjective depends on human determination.

[Quinn]

So, which page is more relevant to my friend's site?

The algo may not be able to determine one of these pages as more relevant than another. The hobby category may be more useful to the user though...

Would the two links pass the same PR value to the action figure collection site?

All links from that page will pass the same PR.

Everybody said that google only calculate those relevant links PR value to a site,

I don't think this is true. The PR which is calculated is independent of the relevancy of the link.

if a link comes from an unrelated site, this link is not a good link to your site, is it right?

Think of it this way. PR is beneficial, but does not determine the relevancy of your site to any particular keyword. If you have an incoming link to your site which is from a page relevant to the theme of your site and a high PR - very beneficial.
If it isn't relevant, but has a high PR - Still good to have....
If low PR and irrelevant - not bad to have but you would need an enormous amount of these links to increase PR, and you would still have to have relevant inbound links...