"Ozymandias" takes the form of a sonnet in iambic pentameter. A sonnet is a fourteen-line poem, whose ideal form is often attributed to the great Italian poet Petrarch. The Petrarchan sonnet is structured as an octave (8 lines) and a sestet (6 lines). The octave often proposes a problem or concern that the sestet resolves or otherwise engages. The ninth line – the first line of the sestet – marks a shift in the direction of the poem and is frequently called the "turn" or, for you Italian scholars, the volta. While the rhyme scheme of the octave is ABBA ABBA, the rhyme scheme of the sestet is more flexible; two of the most common are CDCDCD and CDECDE.
The other major sonnet form is the Shakespearean or English sonnet; it too has fourteen lines, but is structured as a series of three quatrains (of four lines each) and a concluding couplet (consisting of two consecutive rhyming lines). The Shakespearean sonnet is in iambic pentameter and follows the rhyme scheme ABAB CDCD EFEF GG.
Shelley's sonnet is a strange mixture of these two forms. It is Petrarchan in that the poem is structured as a group of eight lines (octave) and a group of six lines (the sestet). The rhyme scheme is initially Shakespearean, as the first four lines rhyme ABAB. But then the poem gets strange: at lines 5-8 the rhyme scheme is ACDC, rather than the expected CDCD. For lines 9-12, the rhyme scheme is EDEF, rather than EFEF. Finally, instead of a concluding couplet we get another EF group. The entire rhyme scheme can be schematized as follows: ABABACDCEDEFEF.
The poem is written in pentameter, meaning there are five (penta-) groups of two syllables in each line. While you've probably heard of iambic pentameter, Shelley's poem makes it really hard to use that designation. Iambic pentameter means that each line contains five feet or groups, each of which contains an unstressed syllable followed by a stressed syllable, as in this line:
half-sunk, a shatt-er'd vis-age lies, whose b>frown (4)
Many of the lines in the poem, however, refuse to conform to this pattern. Take line 12 for example:
No-thing be-side re-mains: round the de-cay
The line begins with a stressed syllable followed by an unstressed syllable; this is called a trochee, and it's the reverse of an iamb. After the initial trochee, we get two iambs, but then we go back to a trochee with "round the," finally ending with an iamb; there's no name for this jumping around! This refusal to conform to any specific meter is evident throughout the poem, and makes it difficult to classify with a simple formula like iambic pentameter.