can't understand flock()

I've scoured over manual page for flock on http://in2.php.net/flock but
I can't understand a few things:

1. will flock wait until a different process releases the lock ?
2. please clarify this line:

" If you don't want flock() to block while locking, add LOCK_NB "

what does "block" mean above?

I'd like to use flock() in a manner that when locking attempt fails on
a file (maybe it is alreadly locked by some other process) then the
script wouldn't wait but go on and do something, eg: close the file.

Thanks
Mike

.

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