You may have installed the "Genieo" or "InstallMac" ad-injection malware. Follow the instructions on this Apple Support page to remove it.
Back up all data before making any changes.
Besides the files listed in the linked support article, you may also need to remove this file in the same way:
~/Library/LaunchAgents/com.genieo.completer.ltvbit.plist
If there are other items with a name that includes "Genieo" or "genieo" alongside any of those you find, remove them as well.
One of the steps in the article is to remove malicious Safari extensions. Do the equivalent in the Chrome and Firefox browsers, if you use either of those.
After removing the malware, remember to reset your home page in all the web browsers affected, if it was changed.
If you don't find any of the files or extensions listed, or if removing them doesn't stop the ad injection, then you may have one of the other kinds of adware covered by the support article. Follow the rest of the instructions in the article.
If you find Apple's instructions too hard to follow, ask for an alternative that doesn't require you to trust a black-box application without knowing what it does.
Make sure you don't repeat the mistake that led you to install the malware. Chances are you got it from an Internet cesspit such as "Softonic" or "CNET Download." Never visit either of those sites again. You might also have downloaded it from an ad in a page on some other site. The ad would probably have included a large green button labeled "Download" or "Download Now" in white letters. The button is designed to confuse people who intend to download something else on the same page. If you ever download a file that isn't obviously what you expected, delete it immediately.
In the Security & Privacy pane of System Preferences, select the General tab. The radio button marked Anywhere should not be selected. If it is, click the lock icon to unlock the settings, then select one of the other buttons. After that, don't ignore a warning that you are about to run or install an application from an unknown developer.
Still in System Preferences, open the App Store or Software Update pane and check the box marked
Install system data files and security updates (OS X 10.10 or later)
or
Download updates automatically (OS X 10.9 or earlier)
if it's not already checked.
