Gmail recently introduced (and forced onto users) this new "Categories" feature. The figure below shows the default Categories that are inserted into your inbox dashboard. Your mail is now categorized by "Primary", "Social" and "Promotions".
The problem with this new feature is that a lot of users including myself either a) already label and/or filter inbox items by type, or b) need to see all inbox items in one overview in order to not miss anything important. Another issue I've come across is that Gmail assumes certain email types are "Promotions" when in fact these emails might have nothing to do with promotions/offers at all and its important you don't miss them. For example, contact form entries from my websites were getting tossed into the "Promotions" category automatically, and this just did not help my workflow at all.
Here's How to Remove or Deactivate Categories:
Your inbox should now be back to its original state, where all emails are classified as "Primary".