Forms of Spam
Spam is usually spread through emails or instant messages, where it usually displays a great offer or that you have "won" something such as a gift card. Pop-ups on websites are almost always spam.
How to Avoid Spam
You can prevent spam from reaching you using spam filters on your email account, and/or not replying to any suspicious emails. This tells whoever's sending you spam that you are an active account that they should keep emailing.