Wa, written with the hiragana for ha (は), is a Japanese postposition ("particle"). It is used to identify the topic of a sentence or discussion. For example:

Suzuki-san wa Nihonjin desu.
Mr/Ms. Suzuki is Japanese.
Once you identify a topic with "wa," it stays in the conversation until you identify another topic.

Sumisu-san wa Igirisujin desu. Rondon kara kimashita.
Mr/Ms. Smith is English. He/she comes from London.
Note that once you establish Smith as the topic of the discussion, you need not mention his or her name again: it's obvious who you're talking about. This is the Japanese answer to pronouns: omission.

Now, here's where wa gets to be tricky. Many people who take Japanese 101 start assuming that it tells you what the subject of the sentence is, but that's the job of another particle, ga. It's perfectly possible to hear something like this:

Watashi wa supagetti desu.
It looks like it's saying "I am spaghetti," and without a context, it could be interpreted that way. What it's really saying is "Me, spaghetti": it would be found when a group of people order dinner, and the "...would like to order..." part of the sentence is implied by default.

You can also have a sentence like this:

Nihon wa joshi ga kawaii desu.
In this sentence, Nihon "Japan" is the topic, and joshi "girl" is the subject of the sentence. The sentence means "Japan? The girls are cute," and it would be a likely answer to a sentence such as "What's so great about Japan?"

The best gloss I've found for wa comes from Basil Hall Chamberlain: "as for." So the above two sentences would be "As for me, it'll be spaghetti," and "As for Japan, the girls are cute." If you're translating long texts, of course, this shortcut will quickly become clumsy, but it's a quick and dirty way to figure out what the wa in a sentence is doing.