What is the difference between Diction and Articulation?

Articulation is the act of expressing something in a coherent verbal form, or an aspect of pronunciation involving the articulatory organs.
Articulation comes from the Latin word for "jointed" or "divided into joints." So it makes sense that the word's original definition described movement at a joint, as in the articulation of your fingers while you're typing. (The joints themselves can also be called articulations.) This noun also describes the act of joining things in such a way that makes motion possible.
the shape or manner in which things come together and a connection is made
expressing in coherent verbal form
Diction is the way you talk, that is, the way you enunciate or pronounce your words, and the words and phrases you choose to use.
If you've seen the musical "My Fair Lady," you know the character Eliza Doolittle struggles with her diction as her mentor tries to teach her to become a lady. Because of her accent, she has the most difficult time properly pronouncing even the most simple phrases, but in the end, she succeeds in saying things like the key phrase, "The rain in Spain stays mainly in the plain."
the manner in which something is expressed in words
Synonyms: choice of words, phraseology, phrasing, verbiage, wording