Opidopecake
Bluelighter
To put it simply -
Agonist - activtae something, turn it on.
Antagonist - block something, turn it off.
Opioid agonists activate or turn on opioid receptors. Opioid antagonists block the receptors preventing opioid agonists from having an effect.
Think of it like a key - agonists are keys that fit perfectly in certain receptors and unlock those effects. Antagonists are like pouring glue into the lock so keys can't get in.
As tri mentioned, buprenorphine is a mixed agonist-antagonist (or partial agonist) - it fits into the receptor like an agonist but only unlocks it part way and then, to use the earlier analogy, breaks off in the lock so other keys can't fit in and further activate the receptor.
This is the most simple and correct definition that has been posted, imho
antagonists block
agonists activate
think of it that way, in a story there is a protagonist (good guy) and an antagonist (bad guy) the antagonist blocks the chemical from activating the receptor in your brain. the agonist activates the receptor in your brain. Both of these occupy the receptor so if you have an opiate antagonist than that chemical will bind to the receptor thus not letting any other chemical bind in that spot, thus making an agonist unable to bind, and unable to activate that receptor. I hope this helps.