Typically politicians depend on political contributions to fund their campaign. This was a problem for Nixon during the Watergate scandal. It came to light that his campaign had a hidden list of illegal donations. There is a great video regarding this, see below:
In 2010, the Supreme Court blocked the ban on corporate spending limits for campaign contributions. They cited the first amendment right to free speech. Direct contributions to candidates are still limited, but the contributions to political parties are fair game.
"The ruling, Citizens United v. Federal Election Commission, No. 08-205, overruled two precedents: Austin v. Michigan Chamber of Commerce, a 1990 decision that upheld restrictions on corporate spending to support or oppose political candidates, and McConnell v. Federal Election Commission, a 2003 decision that upheld the part of the Bipartisan Campaign Reform Act of 2002 that restricted campaign spending by corporations and unions." http://www.nytimes.com/2010/01/22/us/politics/22scotus.html?pagewanted=all
Then comes SUPER PACS:
Super Pacs are political action committees. After 2010, Super Pacs became very popular. They can't donate directly to a candidate, but they can advertise for or against candidates. The donations to super PACs can be unlimited and anonymous.
Donald Trump is running for the republican nominee. He is one of the wealthiest men in the United States and has bragged about the fact that he is self funded. He has also said that he has donated to many politicians in the past and it has helped gain influence with those politicians. See the below short clip from his interview on the late show with Stephen Colbert.
Stephen Colbert even opened his own super PAC: Making a Better Tomorrow, Tomorrow. The comedian sought to show how unlimited funds could be raised and used for any expense in the name of a Super Pac on his satirical television series the Colbert Report. https://en.wikipedia.org/wiki/Colbert_Super_PAC