There are demons in your inbox
While computer users are all given an email account, it's uncommon they are warned about the potential problems that will soon follow. After a year or so, it's probable most of the mail they receive will test their ability to identify "spam". Spam is the name given to unsolicited email that wastes your time. The real spam demons are the emails that kill/damage your computer or steal your identity. I believe this is a must-read-article for new computer users. The information here may save you a trip to our office (to rebuild your system). The information here might also save your grandmother's credit record.
To clarify, I'm not saying email is the only source of problems for computer users. I believe it's one of the most common.
It's probably common knowledge that there are a lot of sick people in the world that somehow find fun in ruining other peoples computers. It's also well known that there are other people who make a living by stealing credit card and other personal information. Your inbox is probably the most common way this cyberterrorism takes place.
Angelic Software
I should start by recognizing the angelic software that helps protect us users from a lot of these dangers. There are free antivirus programs that will scan incoming and outgoing email for viruses. Most email services run intelligent software that intercepts spam before it even makes it into your inbox. There are adware removal tools that help to clean up your computer when it gets infected. Firewalls will help you to control which programs talk to the internet and control how the internet can talk to your computer. It's all great software and everyone should keep their "angelware" up to date. The biggest thing to keep in mind is that none of these programs are perfect. New viruses aren't always detected by the latest antivirus libraries. Some clever spam will make it past the best spam removal filter. Finally, adware removal tools can't always cleanup an infected system. While these tools are helpful the following information still applies.
Basic Spam
Probably the most common form of spam are unsolicited advertisements. These emails will frequently try to target your interests based on your surfing habits. Even if they don't pack some deadly payload I just delete them. You don't want to visit any links in one of these emails. Visiting some random website can lead to your computer running some malicious code that leads to "infection" (I will elaborate below). At the least, your email address will be listed as "alive" and you will be added to countless mailing lists. Even if the email says "click here to remove from mailing list" it's not a good idea (only if you signed up for the mailing list on some reputable site). Visiting web sites is a lot like visiting random homes in New York. For the most part, stick to reputable sites. Use good judgement and remember whats at stake.
While I'm on the topic, you also need to be extremely careful about opening any attachments. I only open attachments when I know the sender and the attachment is expected. Attachments can lead to "infection". Also, some viruses will actually get into your email address book and send out spam emails on behalf of your email account. If you think about this a moment... this means the attached picture of your mom's new dog Daisy might actually be Ebola for computers.
Infections?
Your computer is a finely tuned machine that runs "code". Programs are code, Windows is code, and so are viruses and adware (instances of the more general class of software called malware). You are the only person that can ultimately control whether your computer is running good or bad code. Of course you can never guarantee the bad stuff doesn't run (a good reason to read about backups), for the most part you're in control. Once you are running bad code anything can happen and the outcome can be catastrophic. In the example above I talked about a process that was added to machines to make your own computer send out spam (often illegal content etc). There are an unlimited number of ways bad code can ruin your day. Malware commonly erase peoples hard drives that cause the loss of your data and costly repairs. It can also send your personal information and data to various dark corners of the internet. This can result in an empty bank account, the loss of your new product idea or your identity. There are limitless ways bad code can ruin your life.
Demons in Disguise
The modes of attack via email are countless and evolving. A friend recently shared some random chain letter that urged recipients to forward the mail to everyone in their inbox. Some of these chain mails offer money. Overall, you need to "break the chain" because this is illegal (http://www.ftc.gov/chainmail/). The creators of such emails are gathering addresses or doing something malicious.
My final class of spam is probably the trickiest I've seen so far. At some point you will receive an email from your bank, your credit card, your paypal account, your ebay account, fill in the blank. you will get an official looking email from some site with some urgent message (called phishing). They will ask you to click this link because all of your money has been stolen or something of that nature. No matter how believable the mail is, remember, you are being scammed. You will go to Citibank and enter your account information, this will lead to your financial demise. These cyberterrorists make exact duplicates of official websites and spoof the address links so it's extremely difficult to know when you are at the real bankofamerica.com. These legitimate organizations shouldn't contact you about something like this via email. It's always better to pickup the phone if you are really worried.
This article lists a few of common inbox demons I have seen over the years. Like technology, spam attacks morph and evolve with time. There is a lot more of the above lurking in your inbox and new things to come.