What is the difference between dependency-maven-plugin and maven-dependency-plugin?

Actually, they are the same, it's just that it was moved and renamed. The dependency-maven-plugin is hosted at Mojo while maven-dependency-plugin is hosted at Apache. The recommended plugin to use is the maven-dependency-plugin.

When executing mvn dependency:unpack or dependency:copy from the command line, I get "One or more required plugin parameters are invalid/missing for 'dependency:unpack'"

In order for this to work, you must configure the ArtifactItems as shown here. Note that when executing a plugin from the command line, you must put the configuration tag outside of the executions. If you haven't done this correctly, the error will look like this: 

	 		[0] inside the definition for plugin: 'maven-dependency-plugin'specify the following: