Can someone help?

What is the difference between NM, IM, FM, CM and what do they mean?

NM = National Master, used by some chess federations like USCF.

CM = Candidate Master, anyone with 2200 FIDE can get this title.
FM = FIDE Master, anyone with 2300 FIDE can get this title.
IM = International Master, requires 2400 FIDE at some point and 3 norms (performance at 2450 level)
GM = Grandmaster, requires 2500 FIDE at some point and 3 norms (performance at 2600 level)

There are also women titles:

WCM, anyone with 2000 FIDE can get this title
WFM, anyone with 2100 FIDE can get this title
WIM, requires 2200 FIDE and 3 norms (performance at 2250 level)
WGM, requires 2300 FIDE and 3 norms (performance at 2400 level)

More delails here:

http://en.wikipedia.org/wiki/Chess_title
http://www.fide.com/fide/handbook.html?id=174&view=article

Thankyou so much! Are the ratings on this website very accurate?

They are reasonably accurate within the rating pool of players that they represent.  In other words, you can't compare apples and oranges except to say they're both fruit.  Of course that's not what you want to hear so let's just say your question has been asked many times before and no one really knows.  The consensus seems to be that blitz is slightly deflated and standard is in a state of flux right now thanks to Erik's decision to gift everyone with a rating increase a few months ago.  Try reading a few of these previous threads to see how contentious a topic rating comparison is:

http://www.chess.com/forum/search?keyword=are+ratings+accurate

There's a new hilarious invention by FIDE called FOA titles. FOA stands for FIDE Online Arena. The funny part, beside the names, is that you can also use these titles in over-the-board tournaments.

http://www.fide.com/component/content/article/1-fide-news/8539-fide-online-arena-enters-a-new-era.html
https://www.fide.com/component/handbook/?id=179&view=article