Articulation is the physical act of producing a speech sound. This is affected by how your mouth and tongue are shaped, how strong the muscles in your mouth are and how well you can co-ordinate fine oral movements.
Phonology is a bit like a computer program for producing speech. When you speak a word, your brain has to work out which sounds you will need, what order they go in and how your muscles will need to move to make the sounds. Like all computer programs, this can sometimes get a bit muddled!