1. OTB, yes. Titles obtained vary by national federation.
2. Generally speaking,
NM - 2200 national rating
CM - 2200 FIDE / Youth championships
FM - 2300 FIDE / Youth championships
IM - 2400 FIDE + 3 Norms (You get a norm by playing really well, against strong opposition, basically.). iirc NAYCC (North american youth chess championships) offered IM title to the winner of the whole thing but I could be wrong.
GM - 2500 FIDE + 3 Norms
3. OTB rating measures otb strength. Online ratings measure online strength. If you get matched with a titled player, chances are you guys will have similar online ratings. Thus, chances are that they will play on a similar level of strength to you online. It is likely that they are old, or that they simply do not play online often, and thus have worse board vision online.
As my level has (slowly, painfully) improved on here, I find myself sometimes playing titled players.
There seem to be a few titles: FM, NM, IM, (GM but never played them and likely wont!). And also sometimes there are "W" before these, which I guess means women's player.
Here are my questions
1. How do you get these added to your chess.com account? Over the board play?
2. What is the rating range for each title?
3. Why is it that some players with titles are not that strong? I played some of these titled players and won, quite easily! Is it possible they are retired players who are just getting old?