MCP=active duty with >2yrs of college, but have not graduated yet
ECP=active duty with college degree
MECEP=active duty, remain on active duty, finish college
NROTC=open to anyone (who meets requirements)
OCC=college graduates, one 10wk session of OCS
PLC=college freshmen-juniors, 2 6wk sessions of OCS
Unless you plan on reenlisting, it may be too late to apply for ECP. You have to have already completed your degree at the time of applying. The board only meets a few (2-4) times a year and roughly 10 people get selected on each board. The competition is ludicrous for MCP and ECP.
Not saying you should reenlist or you shouldn't apply, but just be advised it's pretty tough to get into. The MCRC website (OE and ON sections) have newsletters and pages that provide statistics on people accepted for each commissioning program. If I can find the link, I'll send it to you.
I used to plan on applying for MCP but before I met the requirements I got into MECEP. You get commissioned once you finish OCS (unlike me who have to wait until graduation to get commissioned). Also, there is now the MCP-DCP (Meritorious Commissioning Program-Degree Completion Program) for officers who got commissioned under MCP but didn't finish college yet. It's like they're in MECEP but they are already Lt's who have spent a few years in the fleet. Sometimes I think I would have been better off if I would have gotten into MCP and then MCP-DCP. But that's another story.
Anyways, that's a bunch of acronyms to throw out in one post. I spent many years researching all of the programs (and applying for more than one) before I finally got into one. Ask any more questions you have or shoot me a PM and I'll do what I can for you.