The Capitol is the central power of Panem and the largest population in the entire country. The Capitol is the location of the government of Panem, and only citizens of the Capitol are allowed to hold any political offices. The Capitol is a militaristic state, and it is the source of the entirety of Panem's military. It is also the creators of all weapons in Panem. They maintain the Peacekeeping force and the armies that protect Panem from incursion.
The Capitol is run as a democracy that elects the President of Panem every six years. All adult citizens of the Capitol are allowed to vote. The presidential elections take place over the course of a single month ostensibly to prevent any dirty campaigning, but it results in a very high incumbency rate for elected officials.
The Capitol is massive, much larger than any district, and it stretches across huge amounts of land to house its millions of citizens.
There are one-hundred and thirteen cities in the Capitol, not including the Capitol city. Each of these cities has millions of inhabitants, and there are thousands of facilities for military use.
Coriolanus Snow was the President of Panem for thirty years between the forty-fifth Hunger Games and the Seventy-Fourth Hunger Games. He was greatly unpopular with the people of the Capitol, but he was kept in power due in large part to his secret assassination of the various, more qualified, political rivals he had. However, he was ousted after the Seventy-Fourth Hunger Games by Hypatia Glim due to his gross mismanagement and abuse of power.
Hypatia Glim is the current President of Panem. She, unlike her predecessor, is very concerned with the integrity of the Capitol. As such, she plans to benefit all of Panem, rather than just the Capitol in an attempt to quell any rebellion that was growing stronger under President Snow's rule.