Both the Amiga 500 and Amiga 2000 were introduced at the same date. This was caused by their predecessor, the Amiga 100 that was way to expensive and didn't sell very well.
The Amiga 500 was sold for a much lower sellingprice, and being more stable because of the newer Kickstart version next to the new Workbench software.
There are two versions of the A500. The first versions having a OCS-chipset, which was almost identical to the chips used in the Amiga 1000. Later models are equipped with a ECS-chipset, that is a slight different upgraded version of the OCS ones.
The Amiga 500 is very unique because most of the chips are off factory placed in a socket. So upgrading was very easy.
The Amiga 500 has expansionboard on the bottom where you can i.e. place a memory expansion card with a realtime clock, that was backed-up with power coming from a Varta battery. In nowadays many of these batteries are leaking, causing extremely damage to the mainboard and it's components and electrical traces.
On the left side of the Amiga 500 there is another expansionslot, most of the times that is used to plugin a CD-rom drive (A570) or an external harddisk drive.
The next Amiga that was released would be the Amiga 500+, where an onboard realtime clock (RTC) would be standard build-in instead of having to use an expansion card as the biggest difference with the "normal" Amiga 500.