HaShem translates to "the name". It's sort of a nickname for G-d, like the word G-d itself, Jehovah and Adonai. The Jewish people believe HaShem has an actual name and it is hinted at in the "I AM" that people always talk about. Old Jewish customs had it that the high priest was only allowed to speak it once a year during a certain festival but because of the sacredness of it and all the struggles throughout Jewish history, the knowledge of the correct pronunciation has been lost. Basically these "nicknames" are used because Jews are so worried about using His name in vain that they go to extra lengths to make SURE they don't use His name inappropriately.

Don't worry, I've been quick on the Post trigger finger myself.