See:
http://www.nwspeechtherapy.com/functional-articulation-disor...
Fronting: occurs when a back sound (k,g) is produced in the front of the mouth (t,d) ex. "tat" for "cat", "doose" for "goose", "win" for "wing", "sue" for "shoe" (typically disappears by 3 years, 6 months)
Backing: occurs when a front sound (t,d,) is produced in the back of the mouth (k,g). This is less common and typically requires intervention.
Stopping: occurs when airflow sounds (s,f) are stopped ex. "doo" for "shoe", "dat" for "that" (typically disappears by 3 yrs for /s,f/ and up to 5 yrs for /th/)
Final Consonant deletion: occurs when the final consonant sounds are dropped ex. "boa" for "boat", "fee" for "feet" (typically disappears by 3 years, 3 months)
Initial Consonant deletion: occurs when the initial consonants are dropped ex. "eep" for "beep", "ig" for "pig". This is less common and typically requires intervention.
Cluster reduction: occurs when a cluster of sounds are reduced ex. "peak" for "speak", "cape" for "scrape" (typcially disappears by 4 yrs)
Metathesis: occurs when sound sequence is reversed ex. "aks" for "ask", "pasketti" for "spaghetti". This is less common and typically requires intervention.
Gliding: occurs when liquid sounds (l,r) are substituted with glides (w,y) ex. "weg" for "leg", "yeyo" for "yellow" (Typically disappears by 5 yrs)
Weak syllable deletion: occurs when weak syllables are deleted ex. "efant" for "elephant", "tefone" for "telephone" (Typically disappears by 4 yrs)