And how to avoid it.

What is Spam?

Spam is email that is sent to you from advertisers or websites that you gave them your email. It can also be people sending fraudulent content used to make you give up your personal information such as credit card information.
Sorting spam takes up much time when your are using email - sorting between legit emails and spam.
Spam can also be used to spread Malware (look at malware flyer) which can harm your computer.

How to avoid - Use the spam filters built in on most email accounts, DO NOT REPLY to emails that claim to be credit card companies saying that "you need to verify your account" or other information. Have different "one-use emails" or emails that you use for only one use (one for work or school, one for communicating with friends, etc.).