Shoutbox

Here's a question which will probably seem dull for most of the advanced people here, but it's something I need to find out for my informatic class, and until here this is what I got :

1- A Virus is going to "fuck up" other files, yet a worm only needs himself to be able to cause the intented damage.

As stupid as this sounds that's the only thing I got.. How else can I differ them without looking at virus or worm site that will tell me what is which? Like if I'm ona computer and I know I got a dangerous file, how can I know which is it?

Any help is really appreciated.