Dear Suzane,
Apart from this very thread on which you have posted, a few other observations have been made on your very concerns in the following threads:
http://www.cssforum.com.pk/beginners...ualifiers.html
http://www.cssforum.com.pk/beginners...ful-story.html
Kindly go through them and you will find useful information there. However, if any thing specific, you can surely ask so.