The issue triggering this crisis is the impending release, by a U.N. backed tribunal, of indictments concerning the assassination of Rafik al-Hariri, father of the current Prime Minister, Saad al-Hariri. The expectation is that Hezbollah will be named in those indictments, either as assassin or as accessory to Syria. Hezbollah is unwilling to peacefully allow this to happen, hence the Lebanese government falls and fighting may ensue.
Lebanon's multi-group coalition government is the closest thing to a multi-party democracy extant in the Arab world, if you exclude Iraq's U.S.-imposed parliamentary system. Go here to read a Reuters article about the situation.
