If you didn't understand, those spiders are actually a person that typed a word that was in the thread "Another linux question" for example, and the google spider went looking where it could find the word in the web and found it on the thread.
No it's not!
Do you actually think that a search engine trawls the net real time!
In laymens terms
a) I write a brand new website called
www.apj101.com
b) I add a link to my web site in my signature here on CF
c) The google spiders come to CF as they do regularly (lets ignore why for the second)
d) The spiders see my link in my sig and think hmmmm I'll go and visit that site next
e) Spiders come to my site and begin to read EVERY thing i have on my site
f) They copy EVERY peice of text on my site to the google cache (think of this a series of huge computers that contain googles downloaded copy of the entire internet)
e) The spiders will take a note of any external site that i have reference on my web site. e.g.
www.Cromewell.com <- and the spiders say hmmm, we'll visit there later
f) The spiders leave, probably to come back another day to see if anything has changed
Now when you type in a query into google. Google will "search" through the google cache and return your results and links to the page on the web where google found the information in the first place. Sometime google is wrong and the cache no longer matches what is on the real site. Or sometime you click on a link that google suggests and the site no longer exists thus highlighting the disparity between the web and the google cache (ps in that case you can click the little link the google provides under every result called "cache")