The conjunction whereas has a number of meanings, but it’s most commonly used to mean although or while in contrast. In these senses of whereas, the word is grammatically identical to while or although. That is, it introduces a dependent clause.
Whereas often introduces a thought that contrasts with something in the main clause. For example, consider this sentence:
Some couples now both have to work, whereas only one person did before. [Washington Post]
In this sentence, whereas introduces something (the fact that only one person in a couple had to work in the past) that is in contrast with something in the main clause (that now couples both have to work). Notice that whereas functions grammatically in the same way that although or while would. It’s preceded by a comma because it introduces a separate dependent clause. Also notice that it’s not preceded by and or but. And would be unnecessary, and but would be redundant.
Here are a few more examples of whereas used well:
Whereas British Iraq empowered the Sunnis, the Americans would tip the scales in favor of the Shiites. [Wall Street Journal]
The typical prediction was 6500, whereas the index is currently trading at about 5400. [Telegraph]
Whereas 1991 looked completely different from 1971, which in turn could never be confused with 1951, the early nineties are indestinguishable from today. [Boston Globe]