Hi Niwantha,
"Decorum" refers to proper, well-controlled behaviour according to the occasion. Someone who acts with decorum is polite, calm and respectable. It also means the etiquette expected in an event.
Someone who behaves with "dignity" is also polite and worthy of respect. It also describes the personal values of self-respect and respect for others.
Examples:
Even though they are prisoners of war, they should still be accorded basic dignity as humans.
The princess gave a dignified public address.
Fighting back her tears, she acted with commendable decorum (etiquette) while attending her mother's funeral, and gave a eulogy with great dignity (showing great respect for her late mother by showing proper manners).
I hope this helps.