(dialects)Are they just the same laguage spoken in different ways?
Do they have big difference?
Do they just have different accents,but are still English?
Are two people from different regions able to understand each other's dialect?
Are there also many dialects in America and Britain?
No. As you've realised, 'dialect' is the wrong word to use when you are talking about regional variations of English. A dialect is a complete sub-language with its own grammar and vocabulary system. In some parts of the English-speaking world ( e.g. Scotland), people may use occasional dialect words and phrasings within standard English, but the basic grammar and vocabulary of English is the same for everybody.
The word 'also' in your question makes me think that you are comparing English-speaking countries with China. In China, the language spoken in one area may be incomprehensible to inhabitants of another area, but this is not the case with English. There is only ONE English language.
(dialects)Are they just the same language spoken in different ways?
We aren't talking about dialects, but you are right in thinking that all English speakers around the world use 'the same language spoken in different ways'. Accents vary greatly, but the actual words that we say are almost exactly the same.
Is there a big difference?
Not really. Certainly not compared to differences among speakers of other languages.
Do they just have different accents, but are still English?
Yes. If you listen to a farmer from Yorkshire and a farmer from Texas, what the two men say sounds very different indeed. But if you read the subtitles, you'll see that they are actually speaking the same language, and 95% of the words are exactly the same.
Are two people from different regions able to understand each other?
Yes. English speakers can always make themselves understood to other English speakers. If the Yorkshire farmer met the Texan farmer face to face, they might each have to slow down and avoid the use of local words, phrases and slang, and it might take them a few moments to get used to the differences in pronunciation, but they would understand each other and they would be able to have a normal conversation.
Yes, there are a lot of different dialects of English spoken in the US and in Britain. Most speakers can understand all the dialects, but certain dialects can be very hard to understand and require experience or exposure to them to figure them out. Americans generally have an easier time understanding American dialects and British people have an easier time understanding British dialects. The closer your own dialect is to the one you're trying to understand, the easier it is.
The differences between dialects can be between the way they sound, the words they use, and sometimes even the way they put sentences together. Differences in the sounds are called accents - and accents are just one part of dialects.
Here's a cool website about North American dialects of English:
Imagine, in some country (say, Italy) there are many dialects. Not as different as Chinese ones, but still very distinctive and having long separate histories. Some even may deserve to be called 'languages'.
Imagine, they also have a Standard Language, based on one of those dialects (say, the dialect of Tuscany).
Now, a local person would be able to speak both the dialect and the standard. Likely there will remain some regional features in his/her standard Italian. But when we say "she is speaking the dialect now!" we don't mean "she speaks the local variety of standard Italian"!
Many English dialects are extinct now... or have 'leveled' significatnly. That is, many of their distinctive features have diappeared and they are not much different from the standard English today. Then if you are used to speak about English dialects of the past, you might be unwilling to call 'a dialect' the variety which is spoken in the same area today.
It is not because it is technically not a dialect of English. It is because you imagine something like 'Fingallian" when you hear 'a dialect'.
But when English speakin linguists study modern English varieties, they use the term 'dialect'. Or they may say 'variety'.