"Articulation" is the production of speech sounds. Intelligibility is a measure of how well speech can be understood. Someone with an "articulation disorder" can be hard to understand because they say sounds incorrectly. They might substitute one sound for another, e.g., "I hurt my weg"' for "I hurt my leg". They might distort the sound so it sounds funny but is still recognizable as the same sound. Think of how Sylvester the cat from the cartoon says his "s" sounds: "sufferin' succotash". The "s" sounds slushy but is still an "s". The other way to produce a sound incorrectly is to omit it entirely.

"Phonology" is the science of speech sounds and sound patterns. Every language has rules about how sounds can be combined. If a child does not use the conventional rules for his/her language but develops their own, they may have a "phonological disorder". In this case, classes of sounds, rather than single sounds are affected. For example, the child may have their own rule for sounds produced at the back of the mouth - they make them all at the front of the mouth. So instead of saying "cup", they "tup", instead of "go" they say "doe".

Sounds can be classified in three ways: where in the mouth they are produced (place), how they are produced (manner), and whether the voice box is on or off (voice). A "phonological process" is an unusual rule that is being used and changes the place, manner, or voice of a group of sounds. Some phonological processes are: fronting, backing, gliding, cluster reduction, devoicing, stopping.

Adults and children can have articulation problems. Children who do not receive speech therapy and do not "outgrow" their speech difficulties will continue to make speech errors as adults.