Junk (or "spam") email is another name for unsolicited email, usually sent to thousands of collected email addresses simultaneously as part of a sales/marketing campaign. Certain companies make a living from harvesting email addresses in many different ways and selling these lists on for use in these campaigns.
There are a number of ways your address can end up on spam lists - see the
link below to read the article "How do spammers harvest email addresses?"
The harvesting process really can be quite easy. It just takes one person who has your email address in their address book becoming infected with a virus for your address to become public knowledge.
If an account is already receiving spam there are 3 options:
This would include recent versions of Microsoft Outlook, which have a built in Junk Mail folder.
The free Mozilla Thunderbird email client also has built-in junk mail filtering, although it doesn't have the feature range of Outlook and tends to be less stable.
Applications called spam filters can be plugged into most common email clients. These applications each have their own ways of recognising and dealing with junk email - usually by moving it to a temporary 'junk mail' folder. There are both free and paid for filters available, which you can find easily through simple searches of Google. We cannot rate or endorse these third party products, and we also cannot give technical support for problems caused by their installation.
This is the nuclear option! It is a good way to cut all junk mail dead, but isn't always practical, especially if your address is already well publicised. Unfortunately, a well-publicised email address is also one which is very likely to appear on a junk email list.