Rapid Automatic Keyword Extraction (RAKE)

A list of "stop words" are first removed from the input.

Prose is then broken in to "phrases" buy splitting every time you see a phrase delimeter.

Create a matrix of how often n-grams occur with one another, such as "keyword extraction" (a bigram.)

Score the n-grams.

Take the top N n-grams until you have as many keywords as you were hoping to get.