Status and corpus planning are two aspects of language planning. Let's begin with a definition of the latter. Language planning refers to a set of choices made by educational institutions, government, mass communication, and other authoritative bodies to influence how language is used in society.
Status planning is the element of language planning that focuses on the external use of language. It determines which language(s) or dialect(s) will be “official” in a society, in schools, in mass media, in the government, and in courts of law, for instance. For example, Martin Luther translated the Bible into High German, spreading that dialect throughout Germany and eventually making it into the standard German people use in government, schools, and media today. If Martin Luther had spoken Low German and used it for his translation, that may have become the “status” dialect instead.
Questions of an “official” language of the United States also fall under the category of “status planning.” Should that official language be English? Should there by several official languages? Or should there be no official language at all but rather communication in the languages spoken in various regions?
Corpus planning, on the other hand, deals with the internal structures of language and seeks to standardize spelling, vocabulary, pronunciation, and grammatical elements. The effects of corpus planning are visible in the dictionaries and grammars of every language or dialect. Why do Americans use the word “check” to refer to a bank draft while the British use “cheque”? Why do Americans say “trunk” of a car and the British “boot”? Why do Americans write “center” and “theater” and the British “centre” and “theatre”? Each of these variations has become standard for its particular version of English.
The German Spelling Reform of 1998 is another example of corpus planning. German linguists and educators regulated everything from comma usage (no longer used to separate clauses joined by certain conjunctions, etc.) to capitalization (nouns in phrases designating time must be capitalized, etc.) to spelling (some words dropped a silent h; some prefixes must be set off by a space; gh became g in certain words, etc.)
While status and corpus planning are usually defined as distinct aspects of language planning, many linguists emphasize that they overlap significantly, for the internal language choices determined by corpus planning often assume the status of “preferred” or even “official.”