I'm assuming you know the difference between unconsciousness, spirit walking, and permadeath.

When you are knocked unconscious, anything you are holding in your hands, and some things from your belt (ie. moneypouch), may be stolen.

When you spirit walk, items from your packs or pockets may be stolen.

When you permadie, any item on your person may be stolen. Any other items will be turned over to your heir who you may choose to play in your next lifetime.

I'm not sure if there has been any information about what level thieves can steal from carts, saddlebags, etc, or steal your mount.