Speech sound disorders are speech disorders in which some speech sounds (called phonemes) in a person’s native language are either not produced, not produced correctly, or are not used correctly.
Most children make mistakes when learning new words and sounds. A disorder occurs when the child reaches a certain age and is still making certain mistakes.

Types of SSDs: 

1. Articulation disorders: Articulation disorders are characterized by substitution, distortion, omission or addition of sounds in words. A child with an articulation disorder will have difficulty learning how to physically produce certain sounds. Ex: the inability for a child to produce the “r” sound, substituting it with “w,” like saying “twee,” instead of “tree.”
2. Phonological process disorder: It is characterised by set pattern of sound errors. A child with a phonological process disorder will have difficulties learning the sound system and may not realize that certain different sounds have different meanings. Ex: replacing the “d” sound with a “g”; saying “dot,” for example, instead of “got.” Children hear the sound distinction in other peoples’ voices, but unaware when they make the distortion. 
3. Mixed Speech Sound Disorders: In some cases phonetic and phonemic errors may coexist in the same person. In such case the primary focus is usually on the phonological component but articulation therapy may be needed as part of the process, since teaching a child how to use a sound is not practical if the child doesn't know how to produce it.

Classification of errors produced by children with speech sound disorders

1. Omissions: Certain sounds are not produced - entire syllables or classes of sounds may be deleted; e.g., fi' for fish or 'at for cat.
2. Additions (or Commissions): an extra sound or sounds are added to the intended word.
3. Distortions: Sounds are changed slightly so that the intended sound may be recognized but sounds "wrong," or may not sound like any sound in the language. Ex: lisp.
4. Substitutions: One or more sounds are substituted for another; e.g., wabbit for rabbit or tow for cow

Causes and Diagnosis

Some speech sound errors can result from physical problems, such as: developmental disorders (e.g. autism), genetic syndromes (e.g. Down syndrome), hearing loss, cleft palate or other physical anomalies of the mouth, illness, neurological disorders (e.g. cerebral palsy)

In most cases, the reason speech sound disorders occur is unknown. Many children outgrow the problem, but those who cannot learn to produce sounds correctly, or do not learn the rules of speech on their own, need intervention.

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When a person is not able to (or not able to correctly) produce a sound of his/her native language it is known as a Speech sound disorder. It may take the form of Omission, Commission, Distortion or Substitution. SSDs are categorised as Articulation or Phonological based on whether there’s a inability to learn sound or physically produce sound. In most cases the disorders are resolved with time, if not a speech-language pathologist should be consulted.