That message will now go to a list of recipients alright. So will all those addresses. Each recipient will now have a list of addresses that he has no business having.
How would you like your address mailed off to a group of people who don't need to know it?
The types of messages that novices tend to mail off to a list of people are things that are funny. And other novices will forward that on to another list of people. So you and your friends addresses go out further into the world.
Now how can that be dangerous? Well, your address is now on dozens of computers. Have you heard of viruses? You know about them causing trouble in your computer, but have you thought about how they spread? They are designed to find email addresses that are anywhere in a computer. I guess that they find even ones that you have deleted! You have on your system lots of addresses in those emails that you recieved from someone who forwarded some humour to a list of recipients. The virus uses those addresses to email itself to all those people.
Have you recieved any emails that turned out to be viruses? This is how they found you.
Microsoft loves this. Well, almost. They love novices to be able to send each other humour, and towards this end they actually encourage sloppy practices and pre-configure your system to facilitate this happy vulnerability.
Some other tricks are avoid using microsoft products. By far the most commonly available email applications are the ones that come with Microsoft Windows. Many viruses will be prepared for these packages. Get an email application designed by some other company. There are plenty of free ones on the net.
The same reasoning applies to web browsers. You can also catch viruses, "worms", "trojans" and other malicious code from the web.
At best, you have exposed yourself and your friends to spam, if not viruses, etc.