PeopleMayFindItUseful. subtle differences by the usage of the defined function.

[pier4r]

if defined( Class['class_name'] ) {
    notify{'entry 1 defined':}
}
else {
    notify{'entry 1 __NOT__ defined':}
}

if defined( 'class_name' ) {
    notify{'entry 1.5 defined':}
}
else {
    notify{'entry 1.5 __NOT__ defined':}
}

The entry 1.5 one seems always true, as the class_name is loaded (puppet has it in the modules folder). While the entry 1 is true only when the class_name is declared. The entry 1 can be a tad more helpful in case of dependecy checks.