Error

2013-04-15 15:10:48 [SEVERE] Could not load 'plugins/ucars.jar' in folder 'plugins'
org.bukkit.plugin.InvalidPluginException: java.lang.UnsupportedClassVersionError: com/useful/ucars/ucars : 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_5_R2.CraftServer.loadPlugins(CraftServer.java:239)
	at org.bukkit.craftbukkit.v1_5_R2.CraftServer.<init>(CraftServer.java:217)
	at net.minecraft.server.v1_5_R2.PlayerList.<init>(PlayerList.java:55)
	at net.minecraft.server.v1_5_R2.DedicatedPlayerList.<init>(SourceFile:11)
	at net.minecraft.server.v1_5_R2.DedicatedServer.init(DedicatedServer.java:105)
	at net.minecraft.server.v1_5_R2.MinecraftServer.run(MinecraftServer.java:381)
	at net.minecraft.server.v1_5_R2.ThreadServerApplication.run(SourceFile:573)
Caused by: java.lang.UnsupportedClassVersionError: com/useful/ucars/ucars : Unsupported major.minor version 51.0
	at java.lang.ClassLoader.defineClass1(Native Method)
	at java.lang.ClassLoader.defineClassCond(ClassLoader.java:631)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:615)
	at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)
	at java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
	at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
	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(ClassLoader.java:306)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
	at java.lang.Class.forName0(Native Method)
	at java.lang.Class.forName(Class.java:247)
	at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:173)
	... 9 more