A phoneme is a unit of sound in a language that cannot be analysed into smaller linear units and that can distinguish one word from another.
Phonemes are often presented surrounded by // in transcription (e.g. /p/ and /b/ in English pat, bat).
So /p/ and /b/ are two phonemes because they can distinguish between the words "pat" and "bat". /c/ is another, because it distinguishes the word "cat" from "pat" and "bat".
Allophones are any of the variants making up a single phoneme.
So, for example, you might pronounce the letter "T" differently in the two words "stand" and "tip". A common test to determine whether two phones are allophones or separate phonemes relies on finding minimal pairs: words that differ by only the phones in question. For example, the words tip and dip illustrate that [t] and [d] are separate phonemes, /t/ and /d/, in English.
A phoneme is the smallest unit of sound in speech. When children first begin to read, we teach that phonemes have distinctive sounds that are represented by letters and that each phoneme carries distinct word meaning. For example, the word 'cat' has three phonemes, 'c' 'a' and 't', but if 'c' is changed for the other phoneme 'mat', the meaning of the word changes.
Allophones are variations in the realization (or sound) of phonemes, like the different pronunciations of the phoneme 't' in the word 'tar' and the word 'letter', where the 't' allophone (sound) is softened to a 'd' sound. Allophones do not indicate a change in the meaning of the word, nor do they indicate a change in the phoneme used to make up the word.
A difference between the two is that changing the phoneme changes the meaning of the word, whereas changing the allophone changes the sound of the realization of word but does not change the meaning of the word.
A phoneme is the smallest distinct unit of meaning in speech. Phonemes go together to make up words. For example, /p/ + /a/ + /n/ = pan. Changing one of the phonemes will make the word have a different meaning. For example, changing the /n/ to /t/ would make the word "pan" change into "pat", a word with a completely different meaning.
Allophones are slight variations of the sound realizations of phonemes. Allophones are often responsible for different manners of giving sound to speech. For example, you could say the word "stop" with an explosive little puff of air at the end as the British do or you could say it by simply closing your lips on the p and not releasing any air as Americans do. In either case, the phoneme has not changed - it is still a /p/, and the word meaning is still the same. Each slight variations in sound realization in how the /p/ is expressed would be called an allophone.