Grails BuildConfig.groovy, difference between build, compile, and runtime?

What's the difference between build, runtime, and compile, in BuildConfig.groovy (1.3.7)

grails.project.dependency.resolution = {    plugins {        build "acme:acme-cache:latest.integration"    }    dependencies {        build "com.foo.bar:foobar:1.0.5"        runtime "org.apache.httpcomponents:httpclient:4.0.3"        compile("com.thoughtworks.xstream:xstream:1.3.1")    }}