What is the difference between i-90 and i-551?

Renew or replace your I-551 Green Card by filing Form I-90.

USCIS states the I-551 is the name of the green card itself. The Form I-90 is used to renew or replace the green card (the I-551). CITATION:I-551; http://www.uscis.gov/i-9-central/acceptable-documents/list-documents CITATION:I-90; http://www.uscis.gov/i-90