Stateful Firewalls

Hi

As far as my knowledge I know three types of firewalls
1) packet filtering
2) application gateway
3) Stateful

I just noticed that Netgear has a SPI (stateful packet inspection) firewall ?

I was confused? Is there any difference betweeen
Stateful Multilayer Inspection Firewall
and
Stateful Packet Inspection Firewall.

statful packet ???????????????????

Thanks in advance