OTB: Over the Board
FIDE International Chess Federation http://www.fide.com/
USCF: United States Chess Federation
BCF/ECF British/English Chess Federation.
The latter three each has its own rating system. Go to their websites to find out more.
can i enter in any of these federations?how?
what is the relationship with chess.com?
Unless you're thinking about moving to the US or England, don't worry about USCF or ECF.
EDIT:
And as for FIDE as far as I know their ratings start at 1600 (very roughly about 1900 turn-based on chess.com), so if you want to join FIDE I'd recommend you wait until you're a lot stronger.
I don't think you can join FIDE. Instead you join your national federation that is a member of FIDE.
To get a FIDE rating you must play in FIDE-rated events. Most probably your national federation also have their own rating that you can get by playing in national events (some of which may also be FIDE-rated but most probably aren't).
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And as for FIDE as far as I know their ratings start at 1600 ...
"Rating Floor
The rating floor will be 1200 from 1.7.2009. It means that a first result giving a rating at least 1200 for an unrated player is taken into account when calculating the unrated player's first published rating."
i want to know whats OTB rating,what is FIDE,USCF and BCF