If You complete the indicated task You'll unlock the corresponding accomplishment.
Anniversary of the Dead (45) - Complete the Final Day, in either Single or Multiplayer, and finish the game.
Hotter Than Hell (25) - Complete the final day of the "7 Days of Hell" unlockable mode.
A Week To Live (25) - Complete 7 days in any game mode without dying.
Can't Touch This (20) - Rack up a 100x multiplier in any game mode.
Friends in Low Places (20) - Save 25 survivors in any game mode.
Interior Decorator (20) - Kill 25 zombies in any game mode using environmental hazards.
Tour of the City (15) - Complete each arena at least once in the same game in any game mode.
More the Merrier (10) - Kill 10 or more zombies at the same time in any game mode.
Gas Powered (5) - Complete an entire arena, getting every kill with the Chainsaw.
Gun Shop (5) - Kill at least one zombie with each weapon type in any game mode.
Environmental Awareness (5) - Kill a zombie using an environmental hazard in any game mode.
Socialite (5) - Play in an online multiplayer game.