To put it simply there is no difference between a Green Card, a Lawful Permanent Residency Card, and an I-551.
I-551 is the official form number for the card,
Lawful Permanent Residency Card is the official and legal name of that document.
Green Card is the common name for the Residency Card. It has been in popular usage for many decades but only over the past few years has the card been issued with an actual green tint.