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.
There are too many differences between the accents referred to as UK accent and US accent, to list all of them here. Even within the UK and the US, accents are different between states, cities and even neighborhoods.
The Americans and the British differ in the way they stress syllables within words; in many words some parts are silent in one and not so in the other.
The difference in accent is specific for most words and is the reason why dictionaries usually have the pronunciations of both the styles mentioned separately with each word.