To know how sounds are produced is not enough to describe and classify them as language units. When we talk about the sounds of language, the term "sound" can be interpreted in two different ways. First, we can say that [t] and [d], for example, are two different sounds in English: e.g. ten-den, seat-seed. But on the other hand, we know that [t] in let us and [t] in let them are not the same. In both examples the sounds differ in one articulatory feature only. In the second case the difference between the sounds has functionally no significance. It is clear that the sense of "sound" in these two cases is different. To avoid this ambiguity, linguists use two separate terms: phoneme and allophone.
The phoneme is a minimal abstract linguistic unit realized in speech in the form of speech sounds opposable to other phonemes of the same language to distinguish the meaning of morphemes and words.
Let us consider the phoneme from the point of view of its aspects.
Firstly, the phoneme is a functional unit. In phonetics function is usually understood as a role of the various units of the phonetic system in distinguishing one morpheme from another, one word from another or one utterance from another. The opposition of phonemes in the same phonetic environment differentiates the meaning of morphemes and words: e.g. bath-path, light-like. Sometimes the opposition of phonemes serves to distinguish the meaning of the whole phrases: He was heard badly - He was hurt badly. Thus we may say that the phoneme can fulfill the distinctive function.
Secondly, the phoneme is material, real and objective. That means it is realized in speech in the form of speech sounds, its allophones. The phonemes constitute the material form of morphemes, so this function may be called constitutive function.
Thirdly, the phoneme performs the recognitive function, because the use of the right allophones and other phonetic units facilitates normal recognition. We may add that the phoneme is a material and objective unit as well as an abstract and generalized one at the same time.