According to the American Speech-Language-Hearing Association (ASHA) website:

Articulation Disorder – An articulation disorder involves problems making sounds. Sounds can be substituted, left off, added or changed. These errors may make it hard for people to understand you.

Young children often make speech errors. For instance, many young children sound like they are making a “w” sound for an “r” sound (e.g., “wabbit” for “rabbit”) or 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. – ASHA – http://www.asha.org/public/speech/disorders/speechsounddisorders.htm

Phonological Process Disorder – 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 in the front of the mouth like “t” and “d” (e.g., saying “tup” for “cup” or “das” for “gas”).

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 is more difficult for the listener to understand the child. While it is common for young children learning speech to leave one of the sounds out of the word, it is not expected as a child gets older. If a child continues to demonstrate such cluster reduction, he or she may have a phonological process disorder. ASHA – http://www.asha.org/public/speech/disorders/speechsounddisorders.htm

On the pages linked at the top, you will find various resources I use when working with articulation.  The resources are organized by sound pairs or phonological process.