During the month of Ramadan, Muslims fast during the hours of daylight and eat in the evenings. After the sun sets the fast is broken with a meal known as iftar - a mini feast with plenty of sweets and dates.
Non-Muslims and guests visiting Islamic countries are not expected to observe the fast, but it's polite to respect local customs. Be discrete if consuming food or water during the day, and avoid smoking in public places.
In the more liberal of the Middle East countries such as Egypt, Tunisia and Morocco, Ramadan can pass almost unnoticed by most tourists, although observance effects tourists more in places like the Gulf states, Syria, and Libya. Shops might close earlier than normal, there could be extra restrictions on the sale of alcohol, and many restaurants will close during the day. Also, keep in mind that museums and sights will close early to allow people to return to their homes for iftar.