Shield Bashing

[MrJello]

Is this basically just an offhand attack for determining attack and damage?

Going TWF Sword and Shield if it matters.

[DesolateHarmony]

It can be an offhand attack, or you can make it the primary hand attack.

[Seranov]

If you attack with just the shield in a round, you can make it at full BAB and damage, just like if you were using any other weapon by itself.

[Skylancer4]

If you only have one attack due to BAB and you use the shield it would not be an off hand attack. If you attack doing a full attack action attempting a TWF routine it or the sword could be the off hand attack (your choice).
If you have multiple attacks (high BAB) and only use attacks from that you can use either of them for those attacks and neither would be off hand. Once you try to get additional attacks beyond those granted by BAB (ie TWF) you have to choose which weapon will be the off hand attack.