I've two location to set my cart promotion rules, what are the differences?
I can set the rules in Conditions and Actions.
The conditions are the rules that must be met for this promotion to apply.
The actions are the rules that govern what changes are made once the conditions are met and to which products.
Below is just a brief example trying to show the difference.
Example: Spend over $100 and get 10% off any shoes you buy.
The conditions here will be: If subtotal is equal or greater than 100
The actions here will be: Percent of Product Price Discount with the discount amount of 10%. The rule here determine which products are affected, so we would have something like Category is one of Shoes (where Shoes is the actual Magento category). This means any product in the cart that is in the category Shoes, will receive a 10% discount.
EDIT: To answer the question better after your edit.
The condition in your example will set the promotional rule as valid when the cart sub total is greater than $50. Once deemed valid, the action rules dictate which products will receive the 10% discount, in your example it would be any product with a price greater than $50.
So in your example, the rule will read:
If cart subtotal is greater than $50, then give 10% discount on products in your cart which have a price greater than $50. If you have the following products in your cart:
The sub-total of the cart will be $80, therefore the condition has been met. The promotional rule will then look at your actions and apply a 10% discount on all products with a price greater than $50. So only the black pants will receive the discount.
A good way of viewing the differences is:
Condition will evaluate the shopping cart as a whole. This determines if the rule will be applied.
Actions will evaluate the products themselves. This determines which products will be affected by the rule.