By default, Gmail automatically categorizes emails based on the email's content into tabs (see screenshot below), such as “Primary”, “Social”, “Promotions”, and “Updates”.

For example, if the content (subject and body) of your message is overly sales related, then Gmail will most likely categorize the email into the "Promotions" tab.

Additionally, based on the message's content, some emails might be marked as spam and moved to the contact's "Spam" folder. For more information see, "Why are some emails marked as spam?"