Yes AmigaClassicRule sums it up perfectly.
The A2000 is an A500 in a desktop case. The A2000 is just more expandable because of the large case and plenty of zorro II slots etc..
Electronically the A500's side expansion port is a Zorro II slot, just male instead of female. I believe it is completely compatible (electronically no physically) to the A2000's Zorro II slots.