Note to Carl Icahn: Now that you've chosen to run a dissident slate of 12 new board members at Yahoo—or so reports say—take a minute and consider that you're hunting a very different animal than you're used to.

Icahn built his $8.5 billion fortune on the back of a simple strategy: Control is a matter of how, not what. In Wall Street People, Charles Ellis quotes Icahn's uncle and early role model Elliott Schnall on his philosophy:

"When Carl told me he was going after undervalued companies, I said, 'What? Why the hell don't you stick to Wall Street? Stick to what you know,'" Schnall said of Icahn's move on Tappan Co. in 1979. "But to Carl, what the company made was basically irrelevant."

That approach served Icahn well for decades, but it doesn't seem to be working for his growing involvement with tech companies.

Icahn Enterprises' investment in Blockbuster, the video store grappling with an online business model, fell $19 million in 2007, a 34 percent drop in fair value. It lost another $9.4 million last quarter.

And his private funds of company investments returned only 1 percent in the first quarter of 2008, dragged down by Motorola, it's largest position. While Icahn was battling to appoint two directors to Motorola's board, he has lost 39 percent this year. Then there's XO Holdings, a telecom concern of which Icahn owns 60 percent: Yesterday, XO hit a five-year low of 54 cents a share.

Even Icahn's few successes in tech have come in spite of, not because of, his activism. Time Warner rose 26 percent in 2006 after Icahn demanded AOL be split off.

Instead, Time Warner tore down pay walls and made the site mostly free, raking in ad dollars. And as Fortune.com's Colin Barr pointed out recently, Icahn clamored for BEA to sell to Oracle for $17 a share. BEA ignored him and got an extra $2.38 a share for its investors.

So as Icahn and his army of dissidents prepares for an assault on Yahoo, he might consider that what the internet company does is actually very relevant. To help him, here's a quick primer on what he should think about.

Culture counts: Why were Yahooligans high-fiving after they turned down a 70 percent premium over their stock? Because they saw the real victory as one over their hated enemy, Microsoft.

Whether he likes it or not, Icahn is about to wade into bad blood that runs three decades deep. The bitter backstory explains why Google took pains to interfere with a Yahoo-Microsoft merger. And it explains why Microsoft probably doesn't want to come back to the table. As Stifel Nicolaus analyst George Askew noted, "Microsoft will not want to be viewed as a conqueror in a battle of mercenaries."

Watch out for Google: Out here in Silicon Valley, people still use words like cooptition and frenemy with a straight face. So Google will help Yahoo with search ads even as it tries to steal clients from its display ads.

And why would Yahoo put up with this? That's right, the old Silicon Valley-Redmond rivalry. Google's ad alliance with Yahoo is better than a poison pill. Ballmer's "Dear Jerry" letter spells it out clearly.

Job loyalty is a joke: Yahoo isn't BEA. It's not in an aging industry like enterprise software, it's in the internet industry, where workers routinely hop over to younger startups. Anyone who's worked on the Web for more than five years has probably already had multiple employers.

Yahoo employees will be watching Icahn's moves very closely. He's more likely to win their loyalty as shareholders in the company if they believe he's on their side. But if they feel he's hurting their brand—and their personal cred—they'll be gone before Icahn can blow another fuse.

And then there are the software developers. They often aren't employees, yet in this age of open-source software, an internet giant can't get very far without their loyalty. Yahoo is staking its turnaround in large part on their creativity. Icahn would be wise not to piss off these programmers, which he will do if he comes across as a greedy bully. They are vital to Yahoo's future success.