Have a listen here: http://alt-usage-english.org/audio_archive.shtml It's much clearer than a written description, isn't it?

If you are worried that learning one of the many different English accents might prevent you from understanding English spoken in other accents or people not understanding you, don't be concerned. English speaking people can generally understand each other without any great difficulty no matter the accent. Sometimes you will need a little time to "tune in" to the accent or you might need to use slightly different vocabulary in some situations but you will soon have no difficulty.

When you say U.K accent, I assume you mean standard southern English accent (or the stereotypical one, if there is such a thing) because there are a huge variety of wildly different accents within the UK. There are of course also a few different types of American accents so it is difficult to generalise. I would say the biggest difference is the pronunciation of vowel sounds; 'a' in particular is a generally more drawn out, longer and nasally sound in the American accent. Their 'r's' are also more pronounced. As Phil said though, perhaps the best way is to hear the difference for yourself by listening to native speakers on youtube.

Look it up on YouTube.

This is an impossible question to answer as there are many very different UK accents. A person who understood no English at all might listen to a London accent, a Liverpool accent and a Glasgow accent, and presume that these three people were speaking different languages. They're not - it's exactly the same language - but the pronunciation is very different indeed. US accents don't vary quite as much, but there are still a number of variations across the USA. Compare an Alabama farmer to a NYC taxi-driver, and you're sure to hear a big difference. That said, we can make a few generalisations. One is the letter 'r'. In North American English the letter 'r' is always pronounced. In standard British English the 'r' is not pronounced if it comes after a vowel. For example, for many British people the word 'car' is pronounced with an open 'ah' sound, as if it were written 'cah'. But please don't assume that everybody in the UK says 'car' like this. In Scotland, Northern Ireland, and parts of the west of England, the 'r' sound is very much in evidence. As a general rule, Americans are more likely to pronounce a word such as 'butter' or 'water' with a 'flap' sound instead of the 't'. This might appear silent, or sound like a soft 'd'. But bear in mind that not all Americans do this, and not all British people pronounce the 't' in these words. There are many regional accents, such as the typical London accent, which also misses out the 't' in 'water' and 'butter', replacing it with a glottal stop. The best thing for you to do is listen to as many examples as possible, and you'll soon get a feel for the general differences in sound.