Who was Jimmy Hoffa, and what’s the significance of where his body is buried?

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Hoffa was the president (from 1957-1971) of the International Brotherhood of Teamsters, the American (and Canadian, hence International) labor union for truck drivers, warehouse workers, and a variety of other professions involved in the freight moving industry. Depending on your source, Hoffa was anywhere from a major contributor to the growth of the union during the post-war era (from 75,000 when Hoffa first joined the union in 1933 to over 2 million at its height) to singlehandedly responsible, and he was well known for deals such as the National Master Freight Agreement, a nationwide contract between union members and companies.

Hoffa's leadership was riddled with scandal, primarily accusations of corruption that never seemed to stick, even when he was the primary target of investigation by the United States Senate Select Committee on Improper Activities in Labor and Management, otherwise called the McClellan Committee; however, the Justice Department refused to take up any of the cases referred to them by the committee due to procedural errors. Despite the committee having audio obtained via wire tap of Hoffa and mobster Johnny Dio discussing their use of fake union chapters to create additional votes for Hoffa, nothing was able to conclusively stick to him, although the previous president, Dave Beck, was eventually indicted for tax evasion. The committee continued their investigations, not just into the Teamsters, but everything else was of lesser priority (especially the corporate investigations), the goal was Hoffa and they kept their eye on him, with Senator McClellan claiming to the New York Times that Hoffa planned to create his own government:

Senator John L. McClellan said today that the plan of James R. Hoffa to bring all transportation workers in the country into a single union would set up a dangerous "super-government."

McClellan kept building his case, even getting Dio to roll on Hoffa (which didn't amount to much, he invoked the 5th Amendment 140 times in two hours), and presented his case to the public in August of 1957, the bad press was enough for the American Federation of Labor and Congress of Industrial Organizations to kick the Teamsters out in December.

Hoffa eventually saw the inside of a courtroom four times between 1957 and 1963, being acquitted twice while the others were declared mistrials. In 1963, Hoffa was indicted and tried for violations of the Taft-Hartley Act (rules regarding how unions can act and how they must disclose their finances), in particular regarding Hoffa being illegally paid a million dollars by a transport company out of Detroit. The trial ended with a hung jury and Hoffa walked.

Then came the 1965 trials. In March, Hoffa was charged with jury-tampering in the previous trial, paying one juror $10,000 and attempting to influence the husband of another juror. This time, Hoffa and his co-defendants lost and Hoffa was sentenced to eight years for jury-tampering. Then in August, Hoffa was in court again for financial malfeasance, fraudulently arranging a loan from the Teamster's pension fund and redirecting a portion of it into his own accounts. Hoffa lost, again, and was sentenced to five years for fraud and conspiracy. Hoffa claimed that the cases against him were a “personal vendetta” being carried out by then-Attorney General Robert F. Kennedy, who had first acted as counsel for the McClellan Committee and later formed his own "Get Hoffa Squad" upon appointment. Hoffa noted:

If this is justice in the United States, then I pity those who haven't the money to pay for an appeal, because this is a railroad job in my opinion.

Ultimately, despite his cases being argued before the US Supreme Court, Hoffa failed to be found not guilty on appeal and was sent to federal prison in 1967. His attempts to continue running the union from prison through the interim-president fell flat and he ultimately retired from the role in June, 1971. However, his sentence was commuted to time served by President Richard Nixon in 1971, on the grounds that until the day his original sentence would have ended he would not be allowed to:

“engage in the direct or indirect management of any labor organization”.

Hoffa attempted to sue on the grounds that he had never agreed to the restriction, but the court found that the president was acting within his authority as the restriction had been due to Hoffa's misconduct. Meanwhile, Hoffa found he no longer carried to weight over the union that he once had, his once international influence now limited to a single union chapter in Detroit. Even the Mafia that had helped him gain power was against him, in particular former Teamster's vice president and Genovese capo Anthony Provenzano, who Hoffa had had an unspecified feud with while both were in prison. After Hoffa's disappearance, the New York Times reported that Hoffa had told a friend that Provenzano had threatened him and his family, in particular:

to pull out my guts or kidnap my grandchildren.

Hit the character limit, breaking this up into two.