LPN (licensed practical nurse)
LVN (license vocational nurse--used in Texas for sure; possibly other states)
The two above are the same thing, essentially.
RN (registered nurse)
APN (advanced practice nurse...who is and is not considered an APN varies by state nurse practice acts)
ADN (associate degree in nursing)
ASN (associate of science in nursing)
The two above are the same thing.
BSN (bachelor of science in nursing)
MSN (master of science in nursing...for this one, you can specialize in your program of study, e.g. clinical nurse specialist, clinical nurse leader, nurse educator, etc.)
PhD (doctorate of philosophy...but not philosophy, per se)
DNP (doctorate of nursing practice)
There are a quite a few certifying organizations.
For emergency nurses: http://www.ena.org/bcen/Pages/default.aspx
For cardiac and critical care nurses:
For pediatrics: http://www.pncb.org/ptistore/control/exams/index
For OB, neonatal, and women's health nurses: http://www.nccwebsite.org/default.aspx
For surgical nurses: http://cc-institute.org/crnfa/about/programs
(I couldn't find any add'l information for this specialty.)
For various specialties: http://www.nursecredentialing.org/default.aspx
(The certification earned from the ANCC is "RN-BC," while the others look like they are very specific certification designations.)
This isn't everything, I'm sure. If anything I've written is incorrect, please reply.