Poetry is characterized by elements such as tone, speaker, setting, form, symbolism and sound. Even though different forms of poetry, such as free-verse and sonnet forms, can be wildly different in concept, the genre is united by similar elements that have similar goals regardless of format.
Poetry is a means of linguistic expression that typically focuses on a specific emotion or experience. Though poems can have different subjects, formats and lengths, they all can be analyzed for specific elements. Tone is the word that describes the poem's overall feeling and presentation. For example, a poem focused on mourning the death of a loved one will likely have a sad or subdued tone, whereas a poem about a more positive event may have a more upbeat, happy tone. A poem's speaker is the person from whose point of view the story of the poem is being told. Using the example of a person mourning the death of a loved one, the speaker would be the person describing their feelings of loss and sadness.
Some elements may be harder to determine than things like tone or speaker, such as symbolism. Some poems use somewhat cryptic or confusing language to tell a story. Sometimes, this language can actually be intended to refer to something else, such as describing grief over loss as an impossibly heavy load to be carried over a long distance.