A worm differs from a virus in that it:

A worm is designed to operate independently and can replicate itself and spread to other systems without the need for human intervention. This characteristic allows worms to propagate rapidly across networks, leveraging vulnerabilities to exploit and infect other machines automatically. Unlike a virus, which typically requires a user to execute a specific program or file to initiate its destructive capabilities, a worm's autonomous nature means it can go from one system to another without any active participation from users.

In this context, the other options do not accurately describe the fundamental nature of a worm. While a virus does necessitate user execution to spread, a worm's ability to operate without human assistance is what sets it apart. Additionally, characterizing worms as less damaging is not necessarily true; their potential for widespread disruption can be significant. Calling a worm a type of browser extension also misrepresents its nature, as worms are malware categorized separately from extensions or plugins that are commonly associated with browsers. Thus, the defining feature of a worm is its capability to function and propagate autonomously.