A banner linking to your site counts as a link to your site as long as the the link isn't using the "no follow" attribute. You need to be very careful if you are considering buying advertising space on another site as Google are apparently clamping down on this behaviour as they see it as unnatural link building.

If it's a high traffic site then it may still be worth while due to the traffic that might be passed along to you, but if you're doing it to increase your PR or SERPs placement then Google can just flick a switch at any time and negate any benefit so it could end up being a waste of money.

High quality one way unpaid links are the Holy Grail of link building and the most difficult to acquire. Review or product comparison sites can often be a good source of these sorts of links especially if the sites in question are focused on the same market that you trade in.

A link doesn't have to be permanent to count, but but helps if it is. All incoming links have some benefit, even those posted in forums and blog comments.

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Lawrence