If they don't rhyme, then a couplet will have a blank line before and after it. Otherwise, obviously, you won't be able to tell the thing's a couplet.
A distich is basically the same thing as a couplet, but while rap, for instance, quite often uses couplets, describing a rap verse form as consisting of distiches will just make you look...odd.
There have been various fashions for using couplets. Chaucer's Canterbury Tales featured them:
This carpenter hadde newe a wyf
Which that he lovede moore than his lyf
|(that's from The Miller's Tale) and Shakespeare liked to use them, especially as a sort of summing up, both in his sonnets:|
But if the while I think on thee, dear friend,
All losses are restored and sorrows end.
and scenes from his plays:
The time is out of joint, O cursed spite
That ever I was born to set it right!
(those are from Sonnet III and Hamlet)
Then there came Dryden and Pope, who were perfectly happy to write whole books full of the things. In Pope's case they are works of the sharpest possible wit:
'Yet Chloe sure was formed without a spot'-
Nature in her then erred not, but forgot.
With every pleasing, every prudent part,
Say, what can Chloe want?'- She wants a heart.
After that fashion started branching out a bit, but the couplet has never died altogether. Browning ends Apparent Failure:
That what began best, can't end worst,
Nor what God blessed once, prove accurst.
Which is interesting but wrong; and a couplet ends Dylan Thomas's Do not go gentle into that good night:
Do not go gentle into that good night
Rage, rage against the dying of the light.
In fact couplets are still all over the shop:
You'll wonder where the yellow went
When you brush your teeth with Steradent.
Oh dear, I've just realised that there's only one way to end this post, and that's with a couplet:
The poet soars, he tramps through bogs
But on and on the couplet jogs.
Well, you can't say I don't show willing, can you.
Word To Use Today: couplet. The word couplet comes from couple, which comes from the Old French word meaning a pair. Before that it comes from the Latin cōpula, which means bond.