How is spell checking done in search engines?

Hi all,
I was wondering how the search engines provide “Did you know”/“Spelling Correction” to the queries we type at such a fast rate and such a good enough accuracy. They not only correct spelling at word level but also do the same based on Context. EG : “java fle” is given “Did you mean : java file” but when query is just “fle” you get results for this query. Does any one know the basic algorithms that they might be using ?