What's the difference between a phoneme and allophone and can I have some examples?

A phoneme is a contrastive, distinctive, minimal unit of sound. 'Contrastive' because every phoneme is distinct and qualitatively different from other, that means it exists being different from other similar looking sounds; 'Minimal' because it is the smallest unit of sound, unable to be further sub-divided into smaller segments of sound. Sounds m, n, p, b, t, k, etc. are phonemes because they are distinctively different from each other; one cannot be confused for the other. What I mean by distinctive is that replacement of a phoneme in a word by other phoneme will bring change in the word, and in most cases, in its meaning in that language. Phoneme /m/ in 'man' if replaced by /f/ gives us the word 'fan' and so on. English language has 44 phonemes, out of which 24 are consonants, 12 pure vowels and 8 diphthong sounds (phonemes). Technically, phonemes in isolation are written within slant lines. An allophone is a variation of a phoneme in a particular language which is not distinctive but which occurs in a different environment. Environment is the position of (initial, middle or end) and what comes before and after a phoneme in a word. For example, in 'man' phoneme /m/ is in initial position, followed by vowel phoneme /a/. In English, consonant phonemes (/p/, /t/, /k/) have their corresponding allophones in aspirated form ( /ph/, /th/, /kh/). For example, in the word 'paper' the initial /p/ is aspirated (/ph/) but the second occurrence of /p/ is unaspirated.   /ph/ is an allophone of the phoneme /p/. The word will not have a different meaning if you don't aspirate the phoneme (and indeed most of the non-native speakers do that), but the aspirated form will never occur in any other position other than initial position. Other phonemes in English with allophonic variations are /ch/, /l/ and /r/. For example, look at the pronunciation of 'church', 'lapel', 'reference', 'Titanic', and 'cascade' in a phonetic dictionary and compare the form of initial /ch/, /l/, /r/, /t/, and /k/ and the second occurrence of the same phoneme. You will find they are different. But, not aspirating the initial occurrence doesn't change the meaning of the word. But, of course, you cannot aspirate the second occurrence even if you try. You will sound funny if you say peiphr. That is because the allophone occurs only initially, not in any other position.

Allophonic variations are language specific. Aspirated /p/, /t/, /k/, /d/, /b/, etc. in Hindi are distinct phonemes, not allophones of the same phoneme. And that is the case with many other languages too.