An ELT Glossary : Aspiration
Put your hand in front of your mouth and say pin. As you say the /p/ you should feel a small puff of air hit your hand. This (the puff of air following the sound) is aspiration.
Now do the same thing, but say spin instead - and you'll feel no puff of air. The /p/ in pin is aspirated, while the /p/ in spin is not. However, whether the aspirated version of the phoneme is used in the word or the non-aspirated version, there's never any change in meaning - try saying spin but aspirate the /p/. It sounds a bit odd, but there's no chance of someone understanding a different word. The aspirated and non-aspirated versions of /p/ are therefore allophones of the same phoneme (ie variants that never cause a meaning change) rather than two different phonemes, and in phonemic script will both be written as /p/ - giving /pɪn/ and /spɪn/. This is always true of aspiration in English.
For phonetics, however, the difference in quality is important, and in phonetic script the aspirated variant will be indicated by the symbol [ʰ] - so [ pʰɪn] but [spɪn]