Formerly the beloved Jedi Anakin Skywalker, he would later become transformed into an imbittered half-human, mostly-machine, Sith Cyborg.
The pride of Emperor Palpatine's Empire, the former-Jedi-turned-Sith-Lord, Darth Vader was but a shell of his former self. With promises of power to keep his loved ones from dying, Palpatine convinced Anakin to renounce the Jedi Order and join him instead. This great betrayal by one of their own resulted in the deaths of countless Jedi. However, it was at the hands of his former friend and mentor, Obi-Wan Kenobi, that Anakin was horribly mutillated and forced to spend the remainder of his days inside of the black armor that had become recognized with fear throughout the Galaxy.
In the years following his betrayal and transformation, Darth Vader had did everything he could to ensure the success of //his// Empire. Sometimes this called for the help of others to deal with the Imperial's enemies and also to carry out other discrete missions. For this, Vader often employed bounty hunter's. Of those he worked with Boba Fett was the one known for getting the best results, that is, unless you didn't want your opponents to be disentigrated.
The two men shared a begrudging respect for one another that Boba was unaware had begun several years before when Darth Vader was still known as Anakin Skywalker. The two first met when Anakin had saved Boba from being nearly destroyed by the Dark Jedi, Asajj Ventress. It was probably because of that encounter that Vader sought out the help of Boba Fett to the planet Maryx Minor and steal the head of an alien chieftan belonging to the Ancient Order of Pessimists. The head was said to grant the one who possessed it great power. This mission later resulted in Vader deciding to travel to the planet and eliminate Boba because of the potential threat he represented in possessing such a powerful object. Vader failed in killing Boba because of the others resilience and instead a deeper respect was created between the two.
This respect allowed Boba to say things to Darth Vader that would have otherwise resulted in a force-choke for anybody else with the nerve to speak up to the Sith Lord.
It was Vader who had hired Boba, along with a host of other bounty hunters, to track down Han Solo after he had escaped the Imperial's grasp earlier.