<unk>  is a failure to develop language abilities on the usual developmental timetable.<unk>  is distinct from speech<unk> , in which the speech mechanism itself is the focus of<unk> .  Thus,<unk>  refers specifically to a<unk>  in the development of the underlying knowledge of language, rather than its implementation.
The difference between language and speech can be understood by considering the relationship between a computer program and an output device like a printer.  The software running on the computer (a word processing program, for example) is designed to allow a user to create  content that is stored in the computer.  In order to actually create a physical copy of the file, the computer requires another device: a printer.  The printer takes the file and transforms it into a series of commands which control the movement of a print head, thereby making marks on paper.
This two-stage process is something like the distinction between language (computer program) and speech (printer).  When we want to communicate something, the first stage is to encode the message into a set of words and sentence structures that convey our meaning.  These processes are collectively what we refer to as language.  In the second stage, language is translated into motor commands that control the articulators, thereby creating speech. Speech refers to the actual process of making sounds, using such organs and structures as the lungs, vocal cords, mouth, tongue, teeth, etc.
Because language and speech are two independent stages, they may be individually<unk> ed.  For example, a child may be<unk> ed in speech (i.e., unable to produce intelligible speech sounds), but not<unk> ed in language.  In this case, the child would be attempting to produce an age-appropriate amount of language, but that language would be difficult or impossible to understand.  Conversely, a child with a<unk>  typically has not yet had the opportunity to produce speech sounds; it is therefore likely to have a<unk>  in speech as well.<unk>  is commonly divided into receptive and expressive categories. Receptive language refers to the process of understanding what is said to us.  Expressive language refers to the use of words and sentences to communicate what we think, need, and want.<unk>  is a risk factor for other types of developmental<unk> , including social, emotional, and cognitive<unk> , though some children may grow out these deficits, even excelling where they onced lagged, while others may not.  One particularly common result of<unk>  is<unk> ed or inadequate acquisition of reading skills.  Reading depends upon an ability to code and decode script (i.e., match speech sounds with symbols, and vice versa).  If a child is still struggling to master language and speech, it is very difficult to then learn another level of complexity (writing).  Thus, it is crucial that children have facility with language in order to be successful readers.
Neuroscientist Steven Pinker postulates that a certain form of<unk>  may be associated with exceptional and innate analytical prowess in some individuals, such as Albert Einstein, among others. 
In 2005, researchers found a connection between expressive<unk>  and a genetic abnormality: a duplicate set of the same genes that are missing in sufferers of Williams-Beuren syndrome.
Many reports show there is really no clear evidence that<unk>  that can be prevented by training or educating the medical home visitor or health care professional.  Overall, some of the reviews show positive results regarding interventions in<unk> , but are not curing.  (Commentary--Early Identification of<unk> s, 2005)
Language delay
Nonverbal learning disorder
Mixed receptive-expressive language disorder
Language disorder
Pragmatic language impairment
Mental retardation
Developmental disability
Selective mutism