"You are a girl, whereas I am a boy."
Translation:Tu es une fille, tandis que je suis un garçon.
The comma, between the two parts of the sentence is an indication that you have a main clause "you are a girl" and a subordinate clause "whereas I am a boy".
"whereas" = "tandis que" is the conjunction (link) between the two clauses.
"tandis que" is similar to many conjunctions in its building with "que" mandatory.
If you want to know more on this matter: http://french.about.com/od/grammar/a/conjunctions_5.htm