For a company that everyone seems to be talking about, Trader Joe's keeps awfully silent. A new article on the grocery chain that puts the "private" in "privately held" is creating buzz among readers, and reminding investors in the grocery sector just how cutthroat their business can be.

Fortune magazine recently exposed Trader Joe's -- if only just slightly. Among the surprising factoids in its article: The relatively little-heralded TJ's generates about the same in annual sales as Whole Foods Market (Nasdaq: WFMI), sells about $1,750 in wares per square foot -- roughly doubling what Whole Foods achieves -- and does it all without any debt.

Trader Joe's also takes its secrecy seriously. Germany's Albrecht family (which also runs the Aldi grocery chain) owns Trader Joe's, and avoids speaking to the media about the business.

The Fortune article is great for investors interested in how formidable businesses are built, and how they sustain their competitive advantages. While it doesn't sell a ton of options, what populates Trader Joe's shelves is generally both tasty and inexpensive, as epitomized by the cult favorite bargain wine fondly known as "Two-Buck Chuck." The company's edge also includes the somewhat illusory perception that it's an indie mom-and-pop market.

While most of Trader Joe's merchandise is branded with some variation of the store's name, Fortune dispelled any customer illusions that these goods might come from tiny suppliers. Many big names produce wares for Trader Joe's, although the company swears its suppliers to secrecy; it doesn't want shoppers or rivals to know who makes its products. Fortune did manage to uncover that some of TJ's privately branded pita chips are actually produced by PepsiCo's (NYSE: PEP) Frito-Lay.

In our recent chat with Whole Foods' John Mackey, he listed Trader Joe's as one of his company's most impressive rivals. Across the board, nimble competitors like Trader Joe's apply added competitive pressure to large grocery firms like Safeway (NYSE: SWY), SUPERVALU, and Kroger (NYSE: KR). More general retailers such as Wal-Mart (NYSE: WMT), Target (NYSE: TGT), and even the large drugstore chains also increasingly rely on the grocery segment to draw traffic. Trader Joe's might not offer everything, but the food it does carry is high-quality and cheap, and surely erodes sales for rivals.

Whole Foods is one of the only grocery stocks I feel positive about; I looked at major grocery stocks recently, and thought the organic grocer looked like a rare winner in a largely unappetizing retail space. With robust competitors like Whole Foods and Trader Joe's in the market, investors should beware less innovative grocers, lest these stodgier companies grow increasingly stale with shoppers.