I know if run ./bitcoin-qt, wallet GUI program started.
Then if everything is normal and fine, what will happen when run ./bitcoind?
Bitcoind is simply a so-called "headless" version of bitcoin-QT, so instead of a GUI (like Bitcoin-QT has) it has a command-line interface. So booting up bitcoind will give you a command prompt instead of the GUI bitcoin-qt provides.
I believe that there are no differences ( in terms of what commands are available, configurations etc) other then the GUI being different. (or missing)
Some more information about the two can be found here >
https://en.bitcoin.it/wiki/Running_Bitcoin