Everything you ever wanted to know about Hezbollah but were afraid to ask can be found in The New Yorker. This 2002 two-part piece, by Jeffrey Goldberg, has been re-posted in a prominent position on the magazine's website. Here's a snippet.
Fatah, which is part of the Palestine Liberation Organization, had been firing Katyusha rockets into northern Israel from Lebanon, where it had its main base, and Prime Minister Menachem Begin, on the advice of his defense minister, Ariel Sharon, ordered Israeli forces into Lebanon. The stated purpose was to conquer what had come to be known as Fatahland, the strip of South Lebanon under Yasir Arafat's control, and to evict the P.L.O.'s forces. Sharon, though, had grander designs: to secure a friendly Christian government in Beirut and to destroy the P.L.O. It was not so much the invasion that inspired the Shiites, who were happy to see the South free of Arafat and Fatah. The Shiites took up arms when they realized that Sharon, like Arafat, had no intention of leaving Lebanon.
Hezbollah, with bases in the Bekaa and in Beirut's southern suburbs, quickly became the most successful terrorist organization in modern history. It has served as a role model for terror groups around the world; Magnus Ranstorp, the director of the Centre for the Study of Terrorism and Political Violence, at the University of St. Andrews, in Scotland, says that Al Qaeda learned the value of choreographed violence from Hezbollah. The organization virtually invented the multipronged terror attack when, early on the morning of October 23, 1983, it synchronized the suicide bombings, in Beirut, of the United States Marine barracks and an apartment building housing a contingent of French peacekeepers. Those attacks occurred just twenty seconds apart; a third part of the plan, to destroy the compound of the Italian peacekeeping contingent, is said to have been jettisoned when the planners learned that the Italians were sleeping in tents, not in a high-rise building.
Until September 11th of last year, Hezbollah had murdered more Americans than any other terrorist group—two hundred and forty-one in the Marine-barracks attack alone.
It's worth printing out.
Oh, and here's another good New Yorker read, Jon Lee Anderson's analysis of whether Israel's invasion of Lebanon has styrengthened or weakened Hezbollah. The conclusion is grim.