A phoneme is the smallest unit of speech. It is a single sound and does not necessarily have any meaning.
Phonemes are not the physical segments themselves, but abstractions of them. Phonemes are implemented as phones in parole. An example of a phoneme would be the /t/ found in words like tip, stand, writer, and cat.
A phoneme can include slightly different sounds or phones.Two phones that belong to the same phoneme are called allophones.
For instance, the p sound in the words pin and spin is pronounced differently, but is still considered the single /p/ phoneme.