1) Generally speaking, "wa" is a topic marker, and "ga" is a subject marker. The topic is often the same as the subject, but not necessarily. The topic can be anything that a speaker wants to talk about (It can be an object, location or any other grammatical element). In this sense, it is similar to the English expressions, "As for ~" or "Speaking of ~."
2) Beside being a topic marker, "wa" is used to show contrast or to emphasize the subject.
The thing being contrasted against may or may not stated, but with this usage, the contrast is implied.