Pages dropping from index

[Lonely 888]

last week i had about 480 pages indexed, now day by day they get less and less.. now i've got about 310.. and my google traffic has halved. (not that i get much as am in sandbox)
any ideas why my pages are falling out of the index?

 

[SEO]

Did you try submitting a google sitemap? That usally helps keep your pages indexed.

 

[john872]

What about duplicate content. Do you have duplicate content on these pages or duplicate content on other pages of another site of yours.

Also, do you have a good internal linking structure and also plenty of backlinks.

 

[Gracia]

think it's google fixing itself. One of my forums lost 50k pages, however I notice the pages that are left are current and top 1-2 tier of the site. SO I think it's reindexing things from the top down.

 

[shandaman]

This happens, dont get to upset they will probaly come back on next index

 

[SirMille]

Did you try submitting a google sitemap? That usally helps keep your pages indexed.

What's a site map, and how do you set one up? do they cost?

And what is the thing in the red box called? how do I get one?