If the big honking banner ads on your favorite sites suddenly get less annoying over the coming months, here’s why: Google is buying display advertising startup Teracent.

How is this different from Google’s purchase of display advertising stalwart DoubleClick? As is usually the case, Google buys companies for their technology more than anything else. Whereas DoubleClick provides the best display ad marketplace on the planet, Teracent provides new ways to target display advertising.

Teracent’s technology customizes graphic advertisements based on who is watching them, so that a retailer’s closest store could become part of an ad, for example, or new products could be swapped into a company’s standard ad with minimal effort. Ads can also be changed based on the time of day and a person’s language, the company said.

Most people associate display advertising with shotgun banner branding campaigns, geotargeted at best. Teracent’s further targeting will help Google get a leg up on what CEO Eric Schmidt calls “the next billion dollar business” at Google. It should certainly prompt Yahoo to find a similar company to buy, the same way Google’s purchase of DoubleClick prompted Yahoo to buy Right Media.

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