What is the difference between a UK accent and a USA accent?

Accent in language has two meanings. Firstly, it is word
stress (syllabic stress), phrase stress and sentence stress. This enters significantly
into such things as poetics and sentence intonation. Secondly, accent is the realization
of phonemes in any given variety or dialect: how sounds are pronounced. Considering the
first definition, there are some differences in all three categories of accent/stress
between American English (AE) and British English
(BE). To give two examples, in AE the word
address is accented/stressed
'ad-dress while in BE it is
accented/stressed a-'dress, and
concerning sentence stress/accent, AE speakers may stress non-lexical (non-content)
words, whereas in BE speakers only stress lexical (high content) words. For instance, an
AE speaker might say, "And here Alice
began to get rather sleepy,"
but a BE speaker might say, "And here Alice began to
get rather
sleepy."

Accent most often
refers to realization of phonemes in any given variety or
dialect. For example, /p t k/ are usually realized (i.e., pronounced) with an
aspiration, although some American English realizations may has less aspiration than
most British English realizations, and South African English realizations of /p t k/
have no aspiration at all. For the general populace, the most notable elements of accent
as phonemic realization are usually most audible in vowel
sounds.

General American English (GAE,
GA) realization of vowels has some differences from British
English nonregional pronunciation (NRP), though there are
some significant differences in consonant realization as well. Some major
consonant differences between GA and NRP follow:

(1) GA realizes /r/ when it is in front of a consonant and at the the end of
words /foRk, woRkeR/; this is called rhotic, so GA is
rhotic whereas NRP is nonrhotic and only realizes /r/ when
it precedes a vowel /woRRy, caR is/.
(2) GA realizes a midial (middle
position) plosive /t/ as a voiced /d/ so that words like writer and
rider sound alike, whereas NRP realizes a plosive /t/ in front,
medial and final positions, while GA /t/ in final position is often entirely inaudible
as in bite.
(3) GA often, though not always, drops the
y-glide /j/ from words like new and
tune and studio, realizing these as /nu/,
/tun/ and /studiou/, whereas NRP maintains the /j/ y-glide
producing /nju/, /tjun/ and /stjudiou/.

Some major
vowel differences between the phonemic accents in American
English and British English as present in GA and NRP follow:
(1) Since GA is
rhotic (/r/ realized before a consonant /woRk/),
r-coloring changes the vowel in words spelled (or in NRP,
spelt) with the letter r, as
illustrated by the familiar example of the pronunciation of
Berkeley as /berkli/ in GA and /bokli/ in NRP.
(2) In GA,
the realization of the /a/ vowel as heard in TRAP appears in all
the words with an /a/ vowel in the BATH class, a class that is
still represented in GA by father. Thus while in NRP
laugh is a BATH class word (like
father), in GA it is a TRAP class word.  

(3) Most often in GA there is no distinction between LOT
and THOUGHT class words, so that there is no difference in
realization between words like cot and caught,
whereas in NRP the realizations are different and like caht and
cOught. The accents of GA and NRP are further varied by phonetic
considerations such as glottal stops on /t/, gravely voice and aspirated
voice.