There are many good resources where you can read about the National Debt. A brief explanation of what we are graphing here on the site are:
Public Debt – blue: public debt portion of the US National Debt
The Debt Held by the Public is all federal debt held by individuals, corporations, state or local governments, foreign governments, and other entities outside the United States Government less Federal Financing Bank securities. Types of securities held by the public include, but are not limited to, Treasury Bills, Notes, Bonds, TIPS, United States Savings Bonds, and State and Local Government Series securities.
National Debt – green: total US National Debt
The Total National Debt is the Public Debt, plus the Intragovernmental Holdings. Intragovernmental Holdings are Government Account Series securities held by Government trust funds, revolving funds, and special funds; and Federal Financing Bank securities. A small amount of marketable securities are held by government accounts.