Are geese just big ducks?

A duck is the common name for any species of bird in the Anatidae family, of which geese are one "tribe" (known as Anserini).

Here's a phylogenetic tree that shows just how diverse the Anatidae family actually is:

Even the Anserini tribe can be further broken down into three distinct genera:

• Anser (grey geese - pictured below)
• Branta (black geese)
• Chen (white geese)