"Sent" means the message was successfully sent out from your device into the wireless system.
"Delivered" means it actually reached the device you were sending it to - and you will only get a "delivered" message if it was delivered to another Verizon device, since there's no way to determine whether a device on another carrier actually receive the message; Verizon can "see" it on it's own devices, not on other carriers.
In either case, you can't tell if the recipient actually saw the message, or opened it - only that it made it to the (Verizon) device.
