The most noticeable difference between British English and American English is the pronunciation of words. This difference in accent is the first thing you might notice when traveling within England or speaking to an English person. As in the U.S., regional accents vary greatly as you travel within Great Britain, so there is no way to accurately describe "the British accent." However, according to standard British English, Brits tend to omit the "r" that comes at the end of a word. For instance, "to cry a tear" would sound to an American like "to cry a teah." In this way, it resembles stereotypical Bostonian or New York English. In addition, Brits tend to pronounce certain occurrences of consonants (such as the aspirated "t") differently than Americans, who often default to a "d" sound. One example is the compound word "water bottle," which in American English can sound like "wodder boddle."
Another noticeable difference is the use of words that Americans either don't use or use to mean different things. Among these is nappy (diaper), torch (flashlight), trolly (shopping cart), rubbish (trash), ginger (red-haired), snog (to make out), chuffed (pleased) and bollocks (slang for testicles). Food terms can get tricky as well. "Chips" in British English is "French fries" in American English, and "chips" in American English is "crisps" in British English. In addition, when describing food, Brits will say something is "nice" to mean pleasing or tasty, whereas an American would say "good." Some lexical differences can be quite misleading. For example, never threaten to slap someone's fanny in Britain. "Fanny" in American English is a slang term for "butt," while in Britain it is slang for vagina.
In addition to unique words, Brits have unique idioms and expressions that might be puzzling to Americans. "To chat up" is the American equivalent of "to hit on" or "to flirt with." "The dog's "bollocks" (literally the dogs' testicles) is a phrase of overwhelming approval, as in "This restaurant is the dogs' bollocks!" "Bob's your uncle" is a way to demonstrate completion of an event, such as the Italian-co-opted "Voila!" When you want to say something is not expensive, you can say it's "cheap as chips," since the uniquely British food is available at low-cost on just about every street corner.
Another distinction is perhaps less apparent, as it is only noticeable when reading. Pick up a British newspaper and you will soon notice that some words are spelled differently on the other side of the pond (aside from listing the date with day/month/year as opposed to the American month/day/year.) The American "-ize" or "-yze" ending is changed to "-ise"/"-yse," as in the case of organize/organise, realize/realise, recognize/recognise and analyze/analyse. Some British words containing two L's have only one in American English, such as cancelled, counsellor, fuelled and woollen. Common words ending in "-er" are changed to "-re" in British English: centre, theatre, fibre, litre. American English words ending in "-ense" are changed to "-ence" as in British defence, offence and pretence. Brits add a "u" to some instances of "-or," as in colour, humour and neighbour.