The green bars are reputation and you earn more of it as you post and the longer you have been a member. After a while you will also be able to add to other peoples reputation. I am not sure if you can see this icon
in the lower left hand corner of each post. If you can, you can click on this and add to that posters reputation.
The spider are the bots of search engines. They "spider" their way through the site taking note of the information on the site and sending it back to the search engine. For the most part they are good and welcomed.