A rhyming couplet uses iambic pentameter, and as you know iambic pentameter means ten syllables in a line with five iambs, a couplet, on the other hand can have any metrical pattern, not only iambic pentameter, with rhyming words at the end of the lines. Geoffrey Chaucer was the first poet who extensively used iambic couplet in literature. Let me show you a perfect example from mock epic poem,
“The herd of hertes founden is anoon,
Why nil the leoun comen of the bere,
That I mighte ones mete him with this spere?’…
These hertes wilde, and han hem at hir wille.”
(From Geoffrey Chaucer’s The Legend of Good Women)
Here you see how beautifully each heroic couplet uses end rhyme like “noon and goon,” “bere and spere,” and “kille and wille.” Note that I have underlined the iambic pentameter in these lines.