Google offers multiple types of targeting within the ads for both publishers and the advertisers. Some of these targeting options are - Language, Interest-based, Placement and contextual.
Language targeting means people from specific nations are served the ads within the native language. Interest based ads are given based on the search history of the user who is logged into the google services. In case of placement based, the ads make use of the prime areas of the website and show the effective content. In contextual ads, you see the ads based on your website content, keywords and the theme. Google adsense doesn't allow complete control over all of these to publishers.
For publisher side you can make use of "Section Targeting". When you want to make use of keywords and the content within your website for targeting. In such case you can use the section targeting. All you have to do is add the section targeting comment in the content region.
< ! -- google _ad_ section_ start -- >
< ! -- google _ad_section_ end -- >
Another type of targeting that publishers can control is "Interest based". In order to enable or disable this type of targeting, go to Allow & Block ads section on your dashboard. On this page you'll find the settings for Interest-based Ads Preferences. Here you can enable or disable this section. Most of the websites benefit from the interest based ads. However there are some niche that get affected with the revenue if wrong type of ads are served. So make sure to disable the interest based targeting if your site has low revenue.
Note: You can also make more focused targeting simply by blocking the competitive and irrelevant ads for your website. Though this is more of revenue optimization technique than targeting. Still it benefits if you block such ads.