Clickjacking is an exploit which changes the original intention of a click able content on a website with an unintended activity. This activity can be absolutely anything, it can be coded to download a malicious software or to redirect to another website. This means although the user is clicking the correct link or button, he might be redirected to something which is not meant to be there, which can only lead to problems.
In these kind of situation, the website which contains these kind of malicious code, are normally legitimate websites which has been spoofed or hacked.
The Clickjacking is caused due to flaws in the web browser such as Firefox, Safari, Opera and Internet Explorer. These kind of flaws have been patched in the updated versions of the web browser, but attackers always seem to find a away to bypass them.