Sight Words refer to words that an individual can read instantly or automatically.  Each person has different sight words as we are all exposed to different words.  

A word becomes a Sight Word once you have read it numerous times.  If you read one of your Sight Words, its pronunciation and meaning is automatically recognized when you see the word.  There is no specific number of times you need to decode a word before it becomes a Sight Word.  However,  if you notice that your child needs to sound out a word that they have previously sounded out dozens of times, this might be an indication of a learning disability.  

High Frequency Words (HFW) are words that occur frequently in written texts.  Some of the most common HFWs are "the", "and", and "is".  

Many HFWs are the first words to become Sight Words because we read them so often (this leads to confusion surrounding the definitions too).  

Orthographic Mapping is the term given to the process where we store a word in memory for automatic retrieval.  So, words become Sight Words through the process of Orthographic Mapping.  Many of the first words we orthographically map are High Frequency Words. 

 Does that all make sense?