In its simplest form, think of "whereas" as a fancier, nicer word for "but." It's a conjunction (connecting word, like and, but, so, yet, for), and it talks about the difference between two things. Let's use it in a simple sentence, and compare it with "but."
An apple is red but a lemon is orange.
An apple is read, whereas a lemon is orange.
The meaning is the same. However, "but" is used in simpler sentences, "whereas" would be used in sentences with more complex thought, or academic and business English. Also note that "whereas" has a comma before it, *whereas* "but" doesn't. :) Whereas can be used in the beginning of the sentence (although personally I find it awkward), and "but" cannot be used in the beginning of the sentence.
For example: "The climate in Country A remained the same, whereas the climate in Country B exhibited a variety of weather and climate changes over the course of the study."
Hope this helps!