Answer

The world's strongest beer is currently Sam Adams Utopias by the Boston Beer Company at a whopping 24% alcohol.   Dogfish Dogfish Head made World Wide Stout at 23% and the Boston Beer Company have previously made Sam Adams Triple Bock at 17.5%.   All alcoholic strengths are measured by volume.

Normal beer yeast can't tolerate alcoholic strengths above around 10% and is poisoned.   Remember that alcohol is yeast's waste product.   Super yeast strains are bred and used.   Generally speaking these beers have a massive malty sweetness with strong hop bitterness to balance out the alcohol bite.