SENT means the message has been submitted to the cellular network for immediate delivery.
DELIVERED means the message was delivered to the recipients cell phone.
If a message status remains as SENT, it means that the WinSMS gateway never received a delivery report from the network operator for that message.
The networks do not guarantee delivery reports and they even sometimes turn them off or delay them during periods of network congestion.
It could also mean that there is a delay in delivering the message to the cell phone. For example, if the recipient has his/her cell phone off, the message will remain on the network (with status sent) until the recipient turns their cell phone on, or until the message expires which is normally after 72 hours.
In most cases however, if the status does not change to FAILED but remains as SENT, one can safely assume that the message was delivered to the cell phone.