Website defacement is similar to drawing graffiti on a wall, only it happens virtually. Websites’ appearance change - pictures and/or words are scrawled across the defaced website.
Why Websites are Defaced
Attackers may have different motivations when they deface a website. Political motivation is one. Attackers who are against a government or a particular movement can choose to deface related websites to air their views. Attackers who do this are known as "hacktivists". They may change the content of the defaced website with a picture or a message of their choice.
Other attackers may choose to deface a website for fun - to mock site owners by finding website vulnerabilities and exploiting these to deface the website. These attackers "taunt" the site owners. Similar to hacktivits, these attackers deface a website with a picture or a message of their choice.
In both cases, website owners face some damage to their reputation once their sites are defaced.