Defense games are some of the most popular online flash games played by gamers worldwide today. There is widely considered to be two types: tower defence and regular defense games.
A tower defense game is one where your goal is to try and stop your enemies from crossing the map by building towers, usually with armed guards who will shoot your opposition down, as well as setting other traps along the way for your opponent. This is more strategy based of the two types of games, as obviously, where you put the towers is your main strategic way to control the outcome of the game and turn things in your favor. This is likely the more popular of the two genres of defense offering popular online games such as Gemcraft, Protector, and Bubble Tanks Tower Defense.
In most regular defense games, you basically just stand your ground and attempt to hold up whatever 'fort' you may be in, by shooting down, or throwing grenades and other exploding items at enemies that approach to try and kill you and take over your 'fort'. Many of these games are now following the zombie trend, allowing you to stay where you are and mow down as many zombies as possible, taking a cue from popular console games like Left4Dead available on the Xbox 360.
These can be as simple as standing around an shooting enemies, or as complicated as how many feet away you need to place your guarded towers and booby traps so you don't lose all of your belongings to intruders. Either way, defense games can be some of the most addictive online games out there!