1. Kuiper and Allan Chapter 5.1.5
Dialect variation
Kuiper and Allan Chapter 5.1.5

2. Dialect phonology
Dialects of English can vary on the basis of their sound system.
All of them may be varieties of English but they may not sound the same.

3. Systemic differences
The most radical difference is where one dialect of English has a different phoneme inventory from another.
Scots English has a minimal pair in lock and loch.
The final phoneme of the second word is not a voiceless velar stop but a voiceless velar fricative.
Such differences can come about through a merger where a contrast between phonemes disappears and so the dialect has one fewer phoneme.
In New Zealand English hear and hair are often not a minimal pair.

4. Distributional differences
Dialects can also differ in where a particular phoneme appears.
Many dialects of English differ in where /r/ appears.
It always appears before vowels but not always after vowels.
Rhotic dialects have /r/ in both environments.
Non rhotic dialects have /r/ only before vowels
Most dialects of American English are rhotic but Southern states are often non-rhotic or at least less rhotic.
In the UK, Irish and Scots and south western dialects are rhotic while middle class London dialects are not.

5. Realization differences
How a phoneme is pronounced can differ from dialect to dialect, i.e. the particular allophones of a phoneme may differ.
In Caribbean English the vowel in cat is often pronounced further back than it is in standard British or American English.
man sounds more like mon

6. Selectional differences
Some dialects differ from others in where a particular phoneme appears.
In standard British English the word city ends in an /I/ phoneme but in many other dialects it ends in an /i/ phoneme.