The distinction between language and speech was made by Ferdinand de Saussure, the Swiss scholar usually credited with establishing principles of modern linguistics.
Language is a collective body of knowledge, it is a set of basic elements, but these elements can form a great variety of combinations. In fact the number of these combinations is endless.
Speech is closely connected with language, as it is the result of using the language, the result of a definite act of speaking. Speech is individual, personal while language is common for all individuals. To illustrate the difference between language and speech let us compare a definite game of chess and a set of rules how to play chess.
Language is opposed to speech and accordingly language units are opposed to speech units. The language unit phoneme is opposed to the speech unit – sound: phoneme /s/ can sound differently in speech - /s/ and /z/). The sentence is opposed to the utterance; the text is opposed to the discourse.