Spam refers to junk email that is sent out in mass quantities. On average, three-fourth of the email that is sent everyday is spam. Spammers find email addresses in a variety of ways, most commonly by searching for email addresses listed on websites and by means of computer viruses and hacking.
They can sell the list of addresses to other spammers, and continue to pester people with an endless amount of unwanted emails. A lot of spam is harmless, but some can contain viruses and links to scam websites. Whenever you recognize an email as spam, you should just delete it.
You should also be wary of emails that try to imitate legitimate companies, they will try to steal your passwords and personal information. They may ask you to respond with it, or they may try to trick you into going to a website that is not real. This is known as phishing, and it perhaps the most dangerous type of spam.