Yandex is a browser hijacker that takes control of your browser (Internet Explorer, Firefox , Google Chrome). It is installed along with other harmful softwares (PUP, adwares, ....) and you will notice that it can't be uninstalled using conventional method.
Symptoms of infection:
Yandex will harvest your browsing data and your personal information for advertising purposes (targeted ads).
Installation of the Yandex toolbar.
Search engine and homepage will be modified.
Popup ads are randomly displayed during your browsing session.