Tennyson and other Victorian poets like Robert Browning used more traditional, regular forms that the Romantic poets who came before them. The Victorian Age is often thought of as a "reaction" to the heady experiments of Romanticism. The form of "The Eagle" is very neat and regular. Both stanzas are a series of three rhyming lines, so the poem is written in rhyming triplets. The rhymes are formed from simple, one-syllable words.
The meter of the poem consists primarily of iambs, an iamb being a type of metrical "foot" with one unstressed beat followed by a stressed beat: He clasps the crag with crook-ed hands. The two words "He clasps" make up one foot, and so on. Iambic meters are the most traditional in English poetry. Poems usually have lines made up of five iambs. This meter is called iambic pentameter. "The Eagle," though, has only four feet. That iambic pentameter: always hogging all the attention! At least it must be easier to walk with four feet than five. Sorry – bad pun.
The meter isn't completely perfect, though: that might get boring, even in a six-line poem. The second and third lines each begin with a stressed beat followed by an unstressed beat. The name for this kind of foot is a trochee. We're thinking, "Whoa there, Alfred, don't get too crazy!" Fortunately (we suppose) he doesn't, and the rest of the poem follows the iambic pattern to perfection.