In terms of Catalyst switches -
CatOS only works at L2, although it has enough smarts to be able to have a management vlan that you can connect to for managing the switch. So with CatOS you can only configure L2 config such as STP/VTP/switchport vlan membership etc.
IOS works at both L2 and L3 ie. it contains code for both the setting up of the L2 switchports, L2 vlan database etc. + it also contains the code for L3 routing eg. the routing protocols, L3 interfaces etc.
So as an example that is still applicable today, the 6500 can act as a L3 switch running either -
1) CatOS on the supervisor and IOS on the MSFC. When you configure a 6500 this way you need to configure all L2 config using CatOS eg. STP/VTP/the actual switchports etc.
You then configure the L3 information ie. the L3 SVI's, any routing protocol such as EIGRP/OSPF by logging onto the MSFC and using IOS commands.
This is known as hybrid mode on the 6500 and requires you to know both CatOS and IOS commands. It is not used much any more.
2) IOS for everything. This is known as native mode and allows you to configure all the L2 config and the L3 config using just IOS. This is the most common setup for a 6500.
Jon
