"Ozymandias" by Percy Bysshe Shelley is a poem that utilizes a slightly irregular version of the sonnet form.
The rhythmical pattern of the lines is iambic pentameter. To unpack this, we describe poetic lines in terms of two things, the smallest repeated rhythmical unit of the line (called a "foot") and the number of times the foot is repeated in the line. Shelley's poem uses an iambic foot, meaning that its basic pattern is that of an unstressed followed by a stressed syllable. This pattern is repeated five times in each line, and therefore we refer to the meter as pentameter, a word meaning five (penta-) measures (meter).
As the poem is comprised of fourteen lines in a somewhat regular rhyme pattern, it is a sonnet. However, the rhyme pattern is not completely regular. The first two stanzas are irregular open quatrains (abab acdc), appearing to follow the pattern of an English sonnet, but the final six lines appear a variant on the sestet of an Italian sonnet (ece fef).
Thus perhaps the best description is that this is an iambic pentameter sonnet with a slightly irregular or hybrid rhyme scheme.