Plug-in

From Opera webbrowser wiki

A plug-in is an addition to a piece of software that provides extra functionality, extra security or better speed to that software. Opera supports plugins which can be downloaded across the web.

[edit] Problems with plugins

The main controversy over plugins is that they are mostly not supported by the original publisher of the software, and therefore they cannot be held responsible or provide support if something goes wrong.

In addition to this, plugins do not have to be verified by the software publisher and so the content could possibly be malicious, or have security flaws or bugs which could leave your computer vulnerable to attacks.