The problem you describe is sometimes due to adware on the system which is being used to browse the web, which will inject ads into the pages displayed. Sometimes the adware will monitor someone's browsing habits to display "targeted" ads.
Some applications that have free editions which can be used to check a system for adware/spyware include the following:
Assuming, however, that there is no adware on the client side, but, instead, the ads are being served by the website you are visiting, then what you are seeing may be the result of the website owner having an "affiliate" relationship with some advertising network to get revenue from the website, since ads are the mechanism used by most sites to sustain operation of the sites. Here is some information from the Wikipedia article on Infolinks:
Infolinks is an online advertising platform which offers advertising products for publishers, advertisers and brands. The company's products, including InFrame, InSearch, InTag and InText, were designed to overcome banner blindness. Infolinks In3 (Infolinks Intent Intelligence) platform analyzes text on websites and inserts adverts in the text.
The highlighted words may be "in-text advertising" and the website may be using JavaScript provided by the advertising platform or some other mechanism to alter how the page is displayed to your browser to include the ad links when you visit the site.