Articulation is an individual's ability to accurately produce speech sounds and combine them into words. An articulation disorder involves problems producing sounds. Sounds can be substituted, ommitted, added or changed. These errors may make it difficult for people to understand the child. Young children often make speech errors. For example, many young children substitute the /w/ sound for teh /r/ sound (wabbit for rabbit) or they may leave sounds out of words such as "nana" for "banana". The child may have an articulation disorder if these errors continue past the expected age. Not all sound substitutions and ommissions are speech errors. They may be related to a feature of a dialect or accent. For instance, speakers of African Ammerican Vernacular English (AAVE) may use the /d/ sounf for the /th/ sound (dis for this). This is not an articulation disorder, but is one of the phonological features of AAVE.
Phonology refers to the speech sound system of a language. A phonological process disorder involves patterns of sound errors. For example, substituting all sounds made in the back of the mouth like /k/ and /g/ for those sounds made at the front of the mouth like /t/ and /d/ (saying tow for cow and dum for gum). Another rule of speech is that some words start with two consonants (broom or spoon). When a child does not follow this rule and says only one of the sounds (boom/broom or poon/spoon), it becomes more difficult for the listener to understand the child. It is common for young children learning speech to leave out one of the sounds out of a word, however it is not expected as the child gets older. If a child continues to exhibit such cluster reduction errors, he or she may have a phonological disorder.
Information taken form "Speech Sound Disorders: Articulation and Phonological Processes", American Speech-Language-Hearing Association (http://www.asha.org/public/speech/disorders/SpeechSoundDisorders.htm)
Normal Phonetic Development-http://members.tripod.com/Caroline_Bowen/Table4.htm
Phonological Processes- http://members.tripod.com/Caroline_Bowen/Table2.htm
Intelligibility Table- http://members.tripod.com/Caroline_Bowen/Table1.htm