It's a 'trojan' that's been installed by a piece of dodgy software you've downloaded & installed, which tries to make all your searches go through a different search engine and thus making money from their advertisers rather than those of your preferred search engine.
You'll just have to see whether the default search engine has been changed on your browsers, see if any unknown/unrecognised extensions have been installed, or even have a look through your recently-installed apps to verify they're official apps from official sources and not cracked/hacked apps from usenet/torrents.
You can also check you activity monitor (and login items) to see if there's any unrecognised app listed - and force quit/remove it.