Standard English, also known as Standard Written English or SWE, is the form of English most widely accepted as being clear and proper.
Publishers, writers, educators, and others have over the years developed a consensus of what standard English consists of. It includes word choice, word order, punctuation, and spelling.
Standard English is especially helpful when writing because it maintains a fairly uniform standard of communication which can be understood by all speakers and users of English regardless of differences in dialect, pronunciation, and usage. This is why it is sometimes called Standard Written English.
There are a few minor differences between standard usage in England and the United States, but these differences do not significantly affect communication in the English language.
Please note that most dictionaries merely report on words that are used, not on their grammar or usage. Merely because a word appears in a dictionary does not mean that it is standard. For more on this see Is Your Dictionary Descriptive or Prescriptive?