difference between the chains

Before I answer, I will make a couple of points. Packets directed to your firewall which come from the EXTERNAL network will normally be destined for your firewall's external interface. Likewise, packets directed to your firewall which come from the INTERNAL network will normally be destined for your firewall's internal interface.

are packets coming from the INTERNAL network directed to the linux firewall are the packets being handled by the input chain?

Yes (as long as they are destined for your firewall's internal [not external] interface)

are packets coming ALSO from the EXTERNAL network directed to the linux firewall are the packets being handled by the input chain?

Yes (as long as they are destined for your firewall's external [not internal] interface)

are the packets regardless if it is from the INTERNAL (eth1) or EXTERNAL (eth0) are the things handled by the input?

The key thing to remember: If your firewall does not have to make a routing decision about the packet (i.e. redirect to another interface or host), it will be handled by the INPUT chain.

The key thing to remember: If your firewall does not have to make a routing decision about the packet (i.e. redirect to another interface or host), it will be handled by the INPUT chain.

these words made the point clear.

thanks a lot DaLight.