Every reader knows that there are different ways in which two words can sound alike: rhyme, alliteration, assonance and so on. But not everyone is in the habit of thinking of all these sounds as siblings. In fact, they can all be linked together in a very simple way. Perhaps it is an oversimplification, but for the purposes of English poetry I assume that the average English word is composed of a consonant, then a vowel, then a consonant. (ex. - sun, moon, shout, with, bring, flight) This simplified scheme can be expanded to account for longer words, but it is easiest to think in terms of one syllable beginning and ending with consonants. (Obviously, this system is far less useful for Latin and Spanish!) Then all you have to do is go through all the possible combinations of consonant-vowel-consonant. And here they are!
Alliteration. You know the drill - "field and fountain, moor and mountain." Alliteration is very common in Anglo Saxon poetry, and in English poetry in general. What isn't always emphasized is that alliteration is really a likeness between consonants. When two vowels sound alike, you have assonance. Even if they are both initial vowels. (ex: "angels' age" - Herbert) As many have pointed out, Pope's line "And apt alliteration's artful aid" misses both alliteration and assonance.
Assonance. This is where you get two words with the same vowel sounds. Assonance is even more powerful, to my ear, than alliteration; vowels are the body of speech, as singers know. Some kinds of Spanish poetry use assonance the way we use end rhyme in English. The Song of Roland uses it - check out Dorothy Sayers' translation to get an idea of the effect. And if you listen to any kind of popular music or rap or spoken word poetry, you will notice (maybe to your irritation) that assonance, rather than perfect rhyme, joins one verse to the next. (I'm not really bothered by it myself. Blame my Lorca addiction.)
Skothending. I would have had to call it "final alliteration" if I hadn't come across this fantastic Norse word in some of Hopkins' lecture notes. Skothending apparently means "glancing blow," and that is the effect it gives: a very subtle one. One poem which uses it to great effect is Horace's "Carpe diem" ode. In it, Horace keeps ending words with 's' right where there is a metrical pause: "Tu ne quaesieris - scire nefas; "numeros. ut melius"; "sapias, vina liques". Somehow this enhances the haunting, waltz-like feeling of the meter. A subtle effect, and not very well-known.
Rhyme. For perfect rhyme you need the final stressed syllables to match up in both vowels and consonants. Final assonance can also count as rhyme. Slant rhymes would take a whole other post! A lot of prosody writers put three of my categories under the heading of "slant rhyme," but I think it's more useful to distinguish things like pararhyme and save the term "slant rhyme" for really distant cousins like "tree" and "fray," while allowing that someone who mixes up assonance and pararhyme and such in his line endings is, well, slant-rhyming. Seamus Heaney is a modern master of both kinds of slant-rhyme, the more and the less blurry.
Front-Rhyme. Instead of keeping the end of the word and changing the consonant at the beginning, you keep all of the word from the beginning and change the final consonant. This gives power to "The Wreck of the Deutschland" - "giver of breath and bread" - and to "Altarwise by Owl-Light" ("shape without shade...").
Pararhyme. For pararhyme, you keep the hard consonantal shell of the word but change the vowel. Wilfred Owen kind of owns this one: "Courage was mine, and I had mystery; / Wisdom was mine, and I had mastery." He commonly used it in place of end rhyme, and it was his signature technique. WH Auden has a nightmarish little poem in which a good 50% of the fear is generated merely by the pararyhmes: "reader/rider," "midden/madden," "fearer/farer," and finally - well, read it yourself! Brrr.
Homophone. "Rein/rain/reign," "rose/rose/rows," etc. It's unseldom that you would see "rows" and "rose" together in a line: usually homophones are more powerful, not as puns, but as echoes within a single word. To make up a lame example, "rain of fire," when spoken, could also be heard as "reign of fire."
Why is it important to know these things? Because then you can mix them up! Interlacing these categories successfully takes practice, but it's worth it. For a locus classicus of the technique, see these lines from "The Wreck of the Deutschland":
We lash with the best or worst
Word last! How a lush-kept plush-capped sloe
Will, mouthed to flesh-burst,
Gush!—flush the man, the being with it, sour or sweet,
Brim, in a flash, full!
I don't think I've ever seen a more exhilarating chord of assonance, alliteration, rhyme, pararhyme, and beautifully opposed vowels. I've read that phrase, "lush-kept plush-capped," so many times without ever breaking the charm that holds it so elusively together.
More subtle, but no less beautiful, is this line of Dylan Thomas: "And I rose in rainy autumn and walked abroad in a shower of all my days." Assonance is particularly strong here, with all those 'ah' sounds. The play of sharp, sibilant sounds in "rose," "shower," and "days" cuts the richness of the assonance beautifully. Mmmmm. This is making me hungry. Anyone have plans for Mardi Gras?