I quite don't get what the difference between "send" and "send in" is. Any help?
for ex. "I sent the information you asked me" or "I sent in the information you asked me"
I think people say "sent in" (with no indirect object) when they are thinking of the act as one of submitting information to a system.
"Transmitting" the information requires having in mind a particular recipient who has been waiting for this specific information -- say, a functionary in the Government who you're having a problem with, or a person with a job to offer you.
"Submitting" the information involves having in mind an impersonal mail collection bin where large numbers of identical submissions are merely processed automatically -- entering a sweepstakes, for example, or subscribing to a magazine.
> "I'll send you my resume right away."
> "I sent in a box top (along with my name and address) for my secret decoder ring, but I never got one."
> "I sent her a love letter on February 14."
> "I sent in my tax returns on April 15."