If we take poetry as a medium, the tone is the attitude conveyed within the poem to the reader, either by the poem or implied by the poet. Sometimes, they may be two contrasting tones.
Tone can establish a mood, and can reveal an attitude.
POETRY EXAMPLE
For a more specific example, let us take an excerpt from Robert Browning's Porphyria's Lover, a poem where a guy kills his female lover to 'preserve' their love:
(Line)
That moment she was mine, mine, fair,
Perfectly pure and good: I found
A think to do, and all her hair
in one long yellow string I wound
Three times her little throat around, (40)
And strangled her. No pain felt she;
I am quite sure she felt no pain.
As a shut bud that holds a bee,
I warily oped her lids: again
Laughed the blue eyes without a stain. (45)
And I untightened next the tress
About her neck; her cheek once more
Blushed bright beneath my burning kiss:
I propped her head up as before,
Only, this time my shoulder bore (50)
Her head, which droops upon it still:
The smiling rosy little head,
So glad it has its utmost will,
That all it scorned at once is fled,
And I, its love, am gained instead! (55)
Porphyria's love: she guessed not how
Her darling one wish would be heard.
And thus we sit together now,
And all night long we have not stirred,
And yet God has not said a word! (60)
DICTION
If we analyze this for diction, we can categorize some terms under certain themes:
and there are others if you keep looking.
These terms can reveal a pattern in the speaker's thought process---the creepy speaker aims for his goal of love through murder, and had some conscience when it comes to morality, but who knows where it went?
Using diction like that makes the speaker's personality more advanced. "Tress", "burning kiss", "oped" and so on, illustrate more than a mere "I love you".
TONE
If you read this a few times, you should eventually notice that:
You could tell that (2) has some paradoxical meaning to it, which implies that there was probably a mental conflict within the speaker's head with regards to the murder.
Because of line 60, we might imply that God played a role in the speaker's mental conflict---there is a good chance that the speaker had at least considered that he would be punished if he murdered the lover, but at the same time, wanted to murder her to try to preserve her beauty.
We can take this as a contrast between the speaker's tone and the poet's tone:
Although the speaker ultimately decides upon murder and carries it out, Browning perhaps included the wariness of the speaker's actions, as well as the realization that God didn't "[say] a word" (60), to impose his own message that discourages murdering your loved one.
In other words, for Porphyria's Lover, the poet's tone is quite opposite to the speaker's tone. Therefore, you have two different tones going on in the same poem.
Why didn't he just say "I love you"? Well, it wouldn't have a very nuanced tone if he just said that. In this form, we know more about the speaker, and potentially even the poet's view on the speaker's topic.