In phonetics (a branch of linguistics), a phone is any distinct speech sound or gesture, regardless of whether the exact sound is critical to the meanings of words.
In contrast, a phoneme is a speech sound in a given language that, if swapped with another phoneme, could change one word to another. Phones are absolute and are not specific to any language, but phonemes can be discussed only in reference to specific languages.
For example, the English words kid and kit end with two distinct phonemes, /d/ and /t/, and swapping one for the other would change one word into a different word. However, the difference between the /p/ sounds in pun ([pʰ], with aspiration) and spun ([p], without aspiration) never affects the meaning or identity of a word in English. Therefore, [p] cannot be replaced with [pʰ] (or vice versa) and thereby convert one word into another. This causes [pʰ] and [p] to be two distinct phones but not distinct phonemes in English.
In contrast to English, swapping the same two sounds in Hindustani changes one word into another: [pʰal] (फल/پھل) means 'fruit', and [pal] (पल/پل) means 'moment'.[1] The sounds [pʰ] and [p] are thus different phonemes in Hindustani but are not distinct phonemes in English.[2]
As seen in the examples, phonemes, rather than phones, are usually the features of speech that are mapped onto the characters of an orthography.