[benjaminjado]

I'm new to organic chemistry and trying to figure out resonance structures for urea (NH2)2-CO.
I don't know if there are two or three:

1) The greatest contributor to the character of the resonance hybrid should be the structure with the double bond between C and O.

2)  A second structure has a double bond between C and N, with a (+) charge on the double-bonded N and a (-) charge now on the O.

3?)  If I put a double bond between the C and "nitrogen-two" instead of "nitrogen-one," is that a different structure or the same?  How can I tell?

[azmanam]

Even though it looks the same, they are different.  As proof, if we arbitrarily set the single bond distance to 1, and the double bond distance at.5 (bc double bonds are shorter thansingle bonds), then the resonance hybrid for urea has all 3 bond lengths at apps 0.75.

Your analysis is right on