Question : Problem: Purpose of Various Electrical Plug / Outlet Styles?

i am in the process of ordering a new UPS.

one of our current UPS's is a Powerware 5125 2200.  this UPS has a strange plug (5-20P) and we have a special outlet for it.

the new UPS i am considering has another strange plug ... not the same as the 5125.  this new UPS (2880VA) has a L5-30P style plug.

what is the purpose of these various plug styles?

Answer : Problem: Purpose of Various Electrical Plug / Outlet Styles?

Why there are various plug styles (all NEMA in your case) is a question I just cannot answer.    Because different folks at different times have decided "their' design is better than all the others?  

Anyway, I can give you the naming convention for these plugs:
Your example of L5-30P:

L means the plug will lock to make it harder to accidently dislodge.  No L means it won't lock.

The 1st number (5) is "a combination of voltage / number of poles / number of wires / and whether there is grounding or not".  Unfortunately I don't have the formula they use to come up with the listed number.  If it matters, every NEMA plug I've happen to run across, this 1st number is a 5.

The second number (after the -) is the amperage rating of the plug, in this case 30.

The final letter is a P to indicate plug.  An R would indicate receptacle.

Hope that helps.

APC'd Data Center University has a nice course on Fundamentals of Power.  Pretty basic in some things, but some good info.  If I remember correctly, you can sign up and get 1 free course.  Here's the link:
http://lms.globalknowledge.com/ilearn/en/learner/jsp/clients/APC/customer/login.jsp