Short for Browser Helper Object, BHO is a helper object added to your Internet browser. For example, the Google Toolbar is considered a Browser Helper Object. This add-on enables users to perform Google searches through the toolbar and contains additional features that help improve a user's experience.
Although most BHO's are helpful, some can also be malicious and hijack your browser to visit sites he or she may not want to visit, track your viewing habits, etc. These Browser Help Objects are often considered spyware or malware.