Either manually or automatically (via a software program) adding low value or no value comments to other sites.
Automated blog spam:
Nice post!
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Manual blog spam:
I just wrote about this on my site. I don’t know you, but I thought I would add no value to your site other than linking through to mine. Check it out!!!!!
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cluebag manual spammer (usually with keywords as my name)
As time passes both manual and automated blog comment spam systems are evolving to look more like legitimate comments. I have seen some automated blog comment spam systems that have multiple fake personas that converse with one another.