What is the difference between the private and public sector?

Generally speaking, the public sector refers to government and non-profit organizations that provide social goods and services. The private sector includes all businesses and organizations that are not owned or operated by the government and that are incorporated as a non-profit organization. While both the private and public sectors exist to provide goods and services, the public sector tends to fill in the gaps that the private sector does not adequately take care of. For example, most people are able to buy food and shelter with their own income through private sector businesses. For people who do not have the ability to purchase their own food and shelter at market prices, the government and non-profit organizations provide the resources at lower or no cost.

What is a PAC?

A political action committee (PAC) is an organization that is created in order to raise money to fund various political activities and campaigns. PACs can help candidates by providing campaign volunteers, building support through their member networks, and giving financial contributions. Citizens United v. Federal Election Commission (2010) is an important case heard by the US Supreme Court that helped solidify the existence of PACs in US politics as a means to raise and distribute significant amounts of money for campaign funding.

Why do politicians have an advantage during re-elections if they are already in office?

Sitting politicians, called incumbents, tend to be re-elected at higher rates than their competitors for various reasons. Voters do not always remember who their representatives are, but name recognition is easier to maintain over time, thus giving incumbents an advantange when running for re-election. Incumbents also develop more political savvy after being in office and can react to constituent concerns in a more direct way than other candidates can. Also, when a politician decides to run for re-election, his or her political party will usually help them win the primary election for their party because that seat is considered a "safe seat."