The most coherent explanation I’ve found for understanding the difference between the “wa” and “ga” particles in Japanese comes from Jay Rubin’s book Making Sense of Japanese (originally titled “Gone Fishin’”). Namely, use “ga” when you want to emphasize the previous word, and “wa” when you want to emphasize the following words. Which to use depends on what kind of question you’re asking or answering.

For example, he gives the following answers:

1. Ikimashita. "I went."
2. Watashi wa ikimashita. "Me? I went."
3. Watashi ga ikimashita. "I went."

And these matching questions:

1. Dou shimashita ka. "What did you do?" Or: Ikimashita ka. "Did you go?"
2. Soshite, Yamamura-san wa? Dou shimashita ka. "And now you, Mr. Yamamura. What did you do?"
3. Dare ga ikimashita ka. "Who went?"

And that’s why 「私はケーキです。」 does not mean “I am the cake”, no matter what BabelFish says. Except when it does, of course, but for that, you’ll have to read the rest of the book.