Articulation is a term which refers to making sounds. The production of sounds involves the coordinated movements of the lips, tongue, teeth, palate (top of the mouth) and respiratory system (lungs). There are also many different nerves and muscles used for speech.
When young children are growing, they develop speech sounds in a predictable order. A child with an articulation disorder has problems making sounds and forming particular speech sounds properly. One example is lisping (when s sounds like th). A child may not be able to produce a particular sound, for example the 'r' sound, saying 'wabbit' instead of 'rabbit'. These disorders are generally very specific and need speech therapy to correct the concern.
Often children are diagnosed with an articulation disorder when in actual fact they have a phonological disorder (see below) or even dyspraxia. This can often affect the treatment or outcome of therapy.
Phonology refers to the pattern in which sounds are put together to make words. This means that a child can produce a sound correctly, but may use it in the wrong position in a word, or in the wrong word. For example a child may use the 'd' sound instead of the 'g' sound, and so they say 'doe' instead of 'go'.
Another rule of speech is that some words start with two consonants, such as broken or spoon. When children don't follow this rule, and say only one of the sounds ("boken" for broken or "poon" for spoon).
It can be much more difficult to understand children with phonological disorders compared to children with pure articulation disorders. Children with phonological disorders may confuse several phonological rules. Phonological disorders and phonemic awareness disorders (the understanding of sounds and sound rules in words) have been linked to ongoing problems with language and literacy. It is therefore important to correctly assess a child's speech difficulties so that the child gets the most appropriate treatment.
Courtesy: Royal Children’s Hospital Melbourne http://www.rch.org.au