"Sent" means that they went from your phone to the network. The network probably sent it to the recipient and their system didn't send a "read" notification back to your phone.
"Delivered" means that someone received the text AND their system sent a "read" notification back which got all the way to your phone.
It is in concept similar to "read receipts" on emails. You may or may not get a "read receipt" from the recipient. Personally, I disable that.