Telling summarises a situation and jumps to conclusions often describing feelings, outcomes and consequences. Showing describes that same situation in far greater detail. Digging into how's, why's and where's - explaining the facts - exploring evidence - relating what's happening with very little judgement or assumption.
In showing the writer might say, 'there were 2 red apples on the table and 2 green apples in the bowl. The green she hardly noticed but Jane shuddered as her silhouette flashed against the dark red sheen. That memory, that ruby red sheen, haunted her everywhere.'
In telling the writer might say, 'there were just a few apples, it didn't take many to remind her of her past.'
I'll also use Hamlet's 'To be or not to be,' soliloquy, simply because it's well known and out of copyright.
'Hamlet was overwhelmed with emotion; he couldn't decide if life would be better lived as a meek and miserable wretch or as a murder or if life was even worth living at all. He concluded that dwelling upon the unknown could make anyone a hesitant coward and with that, went about his business.'
To be, or not to be, that is the question:
Whether 'tis nobler in the mind to suffer
The slings and arrows of outrageous fortune,
Or to take arms against a sea of troubles
And by opposing end them. To die—to sleep,
No more; and by a sleep to say we end
The heart-ache and the thousand natural shocks
That flesh is heir to: 'tis a consummation
Devoutly to be wish'd. To die, to sleep;
To sleep, perchance to dream—ay, there's the rub:
For in that sleep of death what dreams may come,
When we have shuffled off this mortal coil,
Must give us pause—there's the respect
That makes calamity of so long life.
For who would bear the whips and scorns of time,
Th'oppressor's wrong, the proud man's contumely,
The pangs of dispriz'd love, the law's delay,
The insolence of office, and the spurns
That patient merit of th'unworthy takes,
When he himself might his quietus make
With a bare bodkin? Who would fardels bear,
To grunt and sweat under a weary life,
But that the dread of something after death,
The undiscovere'd country, from whose bourn
No traveller returns,puzzles the will,
And makes us rather bear those ills we have
Than fly to others that we know not of?
Thus conscience doth make cowards of us all,
And thus the native hue of resolution
Is sicklied o'er with the pale cast of thought,
And enterprises of great pith and moment
With this regard their currents turn awry
And lose the name of action.
Clearly showing gives us more detail. More to read and write. More angles to play with and so many more ways to entangle the audience. Giving this amount of detail is creating the human story rather than just serving up a brief summary.
It invites questions and intrigue, leaving open ends and unsolved riddles.
The key thing about showing is that it presents the audience with an opportunity to interpret the facts and bring their own experience into play. That way, they feel the character's feelings for themselves, rather than simply being told what to feel.
The difference in the quality of communication and engagement using this method cannot be overstated.
You should be showing most of the time because this is how your audience becomes drawn into the world you're creating. If it's current action, imagery or general narrative, then show.
If it's exposition you can get away with either, depending on how important it is.
As both song and poetry is always a condensed language, it's best to show as much as possible. Also, both disciplines have serious time and word count constraints, so you have to remain focused and concise as you explore your subject.
We can use telling when we don't want to get stuck in a character or event or situation for too long.
Using the apples as an example. If two characters meet and there's a conversation or tension between them. Getting too involved with the apples would slow things down too much. If it were the middle of a fight scene, spending time on the relative position and colour of apples can easily stop the action unnecessarily.
Similarly, if a character is about to be hit by a speeding train. We need to decide what's important, is it the relentless butchery of the locomotive or the backstory of this character who we've never met before, we don't care about and is only going to feature briefly ? Sometimes the story just needs to keep moving. So we tell.
Another example is in Hamlet's line : 'to take arms against a sea of troubles.'
In this context we're not going to dwell upon what form our weaponry will take, when the battle might be and exactly what these 'troubles' are ? We just want to get the point across that this not one problem, it is a whole 'sea of troubles'.
Yes, it's a great big mess of problems which itself probably requires exploration, but this is neither the time nor the place. Hamlet doesn't have time or head space to go into the detail of the 'troubles' because it will detract from his main concerns and where his thoughts are leading him.
Take note that in song and poetry to a lesser extent, a considerable amount of telling is in the delivery or performance. How many times have you watched a film and known what is about to happen because the music is telling you how to feel ?
Showing requires, detailed evidence, facts and the full story of exactly what occurred without judgement or nor opinion. Telling is a summary and may include, opinion, consequences and the resulting actions, emotions and attitudes of characters. Showing is nearly always longer and more interesting.