Both conditions look at the value of the items in the cart. They differ in how they treat discounts.
Cart total: after discounts
The value of the items after product and order discounts, such as sale codes, "10% off your order" and Buy X Get Y.
Use it when customers must spend the threshold after their discounts. For example, with "Cart total is at least $150", a $160 cart with a 10% code ($144) does not qualify.
Cart subtotal: before discounts
The value of the items before any discounts.
Use it when discount codes shouldn't affect free shipping. With "Cart subtotal is at least $150", the same $160 cart does qualify.
Compare-at ("was") prices aren't discounts. Both conditions use the price the customer is actually paying.
Shipping and tax
Shipping never counts toward either value.
Tax follows your prices. If your prices include tax (common in Australia, the UK and Europe), tax is included. If tax is added at checkout, it isn't.
Operators
Is at least, Is greater than, Is at most, Is less than, Is equal to.
For "free shipping over $X", use Is at least. Avoid Is equal to for money, because it has to match to the cent.
Other currencies
Amounts are in your store currency and convert automatically. You can also set exact amounts per currency. See Selling in multiple currencies.
Only some products should count?
Use Cost of items from collection instead. See Collection conditions.