Answer:
The total US debt is all the money that the US federal government owes. While the debt held by the public is only the portion of the US debt held by private entities (e.g. private investors or foreign governments).
In the past (around 20 years ago), government debt owned by federal agencies was much higher, over 40%, with social security being the largest debt holder. This is called intragovernmental debt. That means that around 60% of the debt was held by the public. But in recent years this has changed. Currently. intragovernmental national debt has fallen to 23%. This means that the debt hold by the public is more or less 77%.
Even as intragovernmental debt has decreased, the total size of the national debt is not as important as the percentage of the national debt compared to the GDP. Currently, national debt represents 98.3% of the GDP which is the highest level since WWII. This is really something to worry about because the government spends a huge portion of its budget paying interests on existing national debt and the higher it is, the less money available for other expenses. As the national debt increases in relation to the GDP, it is more difficult for the country to pay it back. The said thing is that politicians do not pay for it, taxpayers pay it.