Keats and his Romantic peers almost single-handedly revived the ode form for modern readers with poems like, "Ode on a Grecian Urn," and "To Autumn," among others. The ode is an Ancient Greek song performed at formal occasions, usually in praise of its subject. "Ode to a Nightingale" is a particular kind of ode – a Horatian ode, after the Roman poet Horace. In general, a Horatian ode has a consistent stanza length and meter, while other kinds of odes do not – but we don't need to get too far into the weeds here. Suffice it to say that this poem might be the most famous representative of the ode form as a whole. "Ode to a Nightingale" is notable for being the longest of Keats's six "Great Odes." It is also often considered the most personal, with its reflections on death and the stresses of life.
The poem has eight separate stanzas of ten lines each, and the meter of each line in the stanza, except for the eighth, is iambic pentameter. The eighth line is written in iambic trimeter (ahh! too many prefixes!), which means it has only six syllables per line instead of ten.
Iambic pentameter is the most common meter in English. In iambic pentameter, lines are ten syllables long and an unaccented syllable is followed by an accented one (it sounds like da-DUM da-DUM da-DUM da-DUM da-DUM). For example, check out line 2:
My sense, | as though | of hem|-lock I | had drunk
Keats does an excellent job of keeping the meter pretty regular through the poem, without making it sound awkward or strained. At some places, he even adds a syllable to certain words in order to fit the rhythm, like in line 44:
But, in | em-balm|-ed dark|-ness guess | each sweet
In this line, the "-ed" of "embalmed" is pronounced as a separate syllable; it's an old poetry trick that Shakespeare also used.
You probably also noticed that the poem has a rhyme scheme: ABABCDECDE. Keats experimented with several different patterns of rhyme in his various odes, but there's nothing too complicated about this one.