The Hunters were an antagonist alien species debuting in Deep Space 9 - they were a humanoid species that believed it was honorable to hunt down members of a sentient species known as the Tosk, which they had genetically-engineered specifically for the purpose of hunting down.
The Hunters were warlike and cruel in regards to their prey, yet also showed a strange sense of honor and respect when the Tosks outwitted them, enjoying the hunt and seeing it as a major part of their society.
Indeed they believed that taking a Tosk alive was the greatest shame a Tosk could have and had programmed most Tosks into believing honor came with dying in the hunt.
When a lone Tosk sought refuge aboard Deep Space 9 the cultural differences between the Hunters and Star Fleet as well as an emerging friendship between the Tosk and O'Brien would put the Hunters in conflict with the crew.