Same as the other answers (disabled isn't sent to the server, readonly is) but some browsers prevent highlighting of a disabled form, while read-only can still be highlighted (and copied).
http://www.w3schools.com/tags/att_input_disabled.asp
http://www.w3schools.com/tags/att_input_readonly.asp
A read-only field cannot be modified. However, a user can tab to it, highlight it, and copy the text from it.