Spam is unwanted messaging, often sent in large volumes. In email, it can include unsolicited advertising, scams, or other unwanted content. Legitimate messages can also land in the spam folder by mistake.

Spam, phishing, and misclassification
Phishing can try to trick people into sharing credentials, for example. A phishing email can also be spam. An expected shipping confirmation in the spam folder is an example of possible misclassification. The folder name alone does not tell you what the message contains.
Example from an inbox
An unexpected email pressures you to open a link and sign in. Use a route you already know to access the service instead. Reporting the message as spam sends a complaint to the email provider. Unsubscribing from a newsletter you recognize is a different action.