Sight words are generally considered to be those words that students need to learn by sight because they don't follow regular decoding rules.
High frequency words are those words that students need to know by sight because they appear so often in texts that automatic recognition is helpful. Did you know that 13 words (a, and, for, he, is, in, it, of, that, the, to, was, you) account for over 25% of the words in print and 100 words account for approximately 50% of the words found in textbooks today? (From Kyleen Beers When Kids Can't Read: What Teachers Can Do: A Guide for Teachers 6-12 page 212.) Therefore, helping readers develop command over those high frequency and sight words within text is a very effective strategy for supporting fluency.