The Animal Sniffer Plugin is used to build signatures of APIs and to check your classes against previously generated signatures. This plugin is called animal sniffer because the principal signatures that are used are those of the Java Runtime, and since Sun traditionally names the different versions of its Java Runtimes after different animals, the plugin that detects what Java Runtime your code requires was called "Animal Sniffer".
The Fat Jar Eclipse Plug-In is a Deployment-Tool which deploys an Eclipse java-project into one executable jar.
It adds the Entry "Build Fat-JAR" to the Export-Wizard.
In addition to the eclipse standard jar-exporter referenced classes and jars are included to the "Fat-Jar", so the resulting jar contains all needed classes and can be executed directly with "java -jar", no classpath has to be set, no additional jars have to be deployed.