diffeneces between eth0 et ppp0

Hi,

I am a newby il iptables, and I was wondering what is the diffenrence
between eth0 and ppp0 when modem is plugged on eth0.

For example, my forward rules (to allow my lan (on eth1) to access on the
Internet) must be:
iptables -A FORWARD -i eth1 -o ppp0 -j ACCEPT
iptables -A FORWARD -o eth1 -i ppp0 -j ACCEPT

or

iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT
iptables -A FORWARD -o eth1 -i eth0 -j ACCEPT

or

iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT
iptables -A FORWARD -o eth1 -i eth0 -j ACCEPT
iptables -A FORWARD -i eth0 -o ppp0 -j ACCEPT
iptables -A FORWARD -o eth0 -i ppp0 -j ACCEPT
?

Do I have to care about eth0 or i just do
iptables -A INPUT -i eth0 -j ACCEPT
iptables -A OUTPUT -o eth0 -j ACCEPT
and i only take care of ppp0 and eth1 ?

Thx a lot.