opened DNS servers = danger ?

["Norbert François" <norbertlike () gmail com>]

I was surfing, and I found a page where you can download a descent
list of (recursive) opened dns. Then, I've 2 questions:

-> what means "opened" dns ? 'cause when I travel, I'm still
(sometimes I don't know the dns of the current isp) using my isp's dns
(even if my IP doesn't belong to my ISP).

-> What's the danger of an opened dns ? How to protect ? Is it
dangerous for the end-user ?

Thanks,

norbert