Agonist: Drugs that activate receptors in the brain are termed agonists. Full agonist opioids activate the opioid receptors fully, resulting in an opioid effect. Examples are heroin, oxycodone, methadone, hydrocodone, hydromorphine, morphine, opium, and others.
Antagonist: Drugs that block opioids by attaching to the opioid receptor without activating them. Antagonists cause no opiate effect, and block full agonist opioids. Examples are Naltrexone and Naloxone. Naloxone is sometimes used to reverse a heroin (or other full agonist) overdose.
Buprenorphine is a partial agonist, it has properties of full agonists and antagonists. It both activates the opioid receptors in the brain, but to a lesser degree; and blocks other opioids, while allowing for some opioid effect of its own to suppress withdrawal symptoms and cravings. Partial agonists also exhibit a ceiling effect; beyond a certain dose no further effects are felt.