Final answer:
People use the term 'accent' to refer to pronunciation differences between American and British English, while 'dialect' encompasses broader linguistic variations including grammar and vocabulary. American and British English have many dialects, but accents are what people most commonly notice. Accents such as rhotic (where the 'r' sound is pronounced) and non-rhotic (where the 'r' sound is not pronounced) are distinctive features used to describe these differences.
Step-by-step explanation:
People often say British accent versus American accent rather than British dialect and American dialect because the term 'accent' specifically refers to the way in which words are pronounced, which is what is most noticeably different between American and British English speakers.
On the other hand, the term 'dialect' includes not only pronunciation but also grammar, vocabulary, and syntax, which can vary much more substantially and can be subtler to detect. Although both American and British English have various dialects, when people refer to the accent, they are typically commenting on the distinct pronunciation features that are immediately recognizable, such as the rhotic and non-rhotic pronunciation or the 'rolling' of Rs in Scottish English.
English dialects themselves are complex and encompass a variety of regional and social variants, each with its own set of grammatical, vocabulary and pronunciation characteristics. For instance, North American dialects can be pinpointed with quite a fine level of detail, even to specific cities with their neighborhood-level dialects.
While standard American English is often the dialect taught and used in formal situations, nonstandard dialects can reveal much about a person's cultural and geographic background, with certain expressions and pronunciations linked to specific regions and ethnicities.