Manafort, 68, is a long-time political consultant and lobbyist in Washington.
He helped multiple Republican presidential nominees manage their efforts at their party conventions, including Mr Gerald Ford in 1976, Mr Ronald Reagan in 1980 and 1984, and Mr George H.W. Bush in 1988. He also managed Mr Bob Dole's 1996 presidential bid, The Washington Post reported.
Manafort also worked on behalf of a number of questionable international actors, including Filipino dictator Ferdinand Marcos and Russia-backed Ukraine president Viktor Yanukovych, who was ousted in 2014.
He co-founded the Washington lobby firm Black, Manafort, Stone and Kelly and then later Davis Manafort. In March last year, Mr Donald Trump hired Manafort to help corral delegates for the upcoming convention. He was Mr Trump's campaign chairman until August last year.
Gates, 45, has been labelled Paul Manafort's "protege", his "right-hand man". He interned at Manafort's lobbying firm, Black, Manafort, Stone and Kelly, according to US magazine Mother Jones.
In 2006, he joined Davis Manafort, which worked on Yanukovych's campaign. This work connected Manafort and Gates with Russian oligarch Oleg Deripaska, an aluminium magnate and ally of President Vladimir Putin of Russia.
The indictment says that in 2011, Manafort created another firm, DMP International, and Gates became a key player in both companies.
The New York Times reports that Gates took over Eastern European deals, travelling all over the world to meet potential business partners, developing deals and negotiating contracts.
Gates also joined the Trump election campaign last year, signing on as Manafort's deputy.
Papadopoulos acted as a foreign policy adviser to Mr Donald Trump during his election campaign.
The 30-year-old graduated from college in 2009, before moving to London to get a master's degree in security studies.
He worked for the Hudson Institute in Washington, starting in 2011 as an unpaid intern. The institute told CNN that he provided research "on a contractual basis" to a senior fellow, but he was "never a salaried employee".
He was also briefly a foreign policy adviser to then Republican presidential primary candidate Ben Carson - now Mr Trump's Secretary of Housing and Urban Development - before he joined the Trump campaign.
He is currently an oil, gas and policy consultant, according to his LinkedIn page.
What are the charges?
PAUL MANAFORT AND RICHARD GATES
The indictment against Paul Manafort and Richard Gates says they both generated tens of millions of dollars of income from work in Ukraine and laundered money through scores of US and foreign entities to hide payments from the US authorities between 2006 and through at least last year.
While falling short of providing a smoking gun for top-level conspiracy, the charges point to a potential pattern of top Trump associates looking to Russia and its proxies for political and economic gain, Reuters reported.
In June, Manafort and Gates retroactively registered with the Justice Department as foreign agents, in a Foreign Agents Registration Act filing that showed they earned US$17.1 million (S$23.3 million) for lobbying on behalf of the Party of Regions, a pro-Russian political party in Ukraine, between 2012 and 2014. The party backed former pro-Kremlin Ukrainian president Viktor Yanukovych, who has been living in exile in Russia since he was ousted by mass street protests in Kiev in 2014.
The indictment says Manafort and Gates concealed from the United States their work and revenue as agents of Ukrainian political parties. They used their wealth to lead a "lavish lifestyle" without paying taxes on the income. The indictment says that more than US$75 million flowed through Manafort's and Gates' offshore accounts, Reuters said.
The indictment accused Manafort of laundering US$18 million and Gates, US$3 million.
Manafort and Gates pleaded not guilty to the charges. They were released on bail of US$10 million and US$5 million respectively and placed under house arrest.
The former Trump campaign aide pleaded guilty to lying to the Federal Bureau of Investigation about Kremlin-related contacts, a plea deal revealed on Monday showed.
Papadopoulos, a former Trump foreign policy adviser, admitted that he tried to hide contact with a Moscow-linked professor who was offering "dirt" on Mr Donald Trump's election rival Hillary Clinton. He pleaded guilty to lying about "the timing, extent and nature of his relationships and interactions with certain foreign nationals whom he understood to have close connections with senior Russian government officials".