Bukkit Plugin - Error (from the server.log)

[SEVERE] Could not load 'plugins\Sneak.jar' in folder 'plugins'
org.bukkit.plugin.InvalidPluginException: java.lang.UnsupportedClassVersionError: com/chrisfire56/sneak/Sneak : Unsupported major.minor version 51.0
	at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:184)
	at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.java:305)
	at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:230)
	at org.bukkit.craftbukkit.v1_6_R3.CraftServer.loadPlugins(CraftServer.java:241)
	at org.bukkit.craftbukkit.v1_6_R3.CraftServer.<init>(CraftServer.java:219)
	at net.minecraft.server.v1_6_R3.PlayerList.<init>(PlayerList.java:56)
	at net.minecraft.server.v1_6_R3.DedicatedPlayerList.<init>(SourceFile:11)
	at net.minecraft.server.v1_6_R3.DedicatedServer.init(DedicatedServer.java:107)
	at net.minecraft.server.v1_6_R3.MinecraftServer.run(MinecraftServer.java:393)
	at net.minecraft.server.v1_6_R3.ThreadServerApplication.run(SourceFile:583)
Caused by: java.lang.UnsupportedClassVersionError: com/chrisfire56/sneak/Sneak : Unsupported major.minor version 51.0
	at java.lang.ClassLoader.defineClass1(Native Method)
	at java.lang.ClassLoader.defineClassCond(Unknown Source)
	at java.lang.ClassLoader.defineClass(Unknown Source)
	at java.security.SecureClassLoader.defineClass(Unknown Source)
	at java.net.URLClassLoader.defineClass(Unknown Source)
	at java.net.URLClassLoader.access$000(Unknown Source)
	at java.net.URLClassLoader$1.run(Unknown Source)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(Unknown Source)
	at org.bukkit.plugin.java.PluginClassLoader.findClass0(PluginClassLoader.java:80)
	at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:53)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	at java.lang.Class.forName0(Native Method)
	at java.lang.Class.forName(Unknown Source)
	at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:173)
	... 9 more