Halal" is a Quranic Term, which means 'Lawful' or 'Permitted'. Sometimes it is translated as 'Acceptable' or 'Not forbidden'. Halal foods and drinks are permitted for consumption by Allah- the supreme lawgiver. In the Holy Quran, Allah commands Muslims and all of mankind to eat of the Halal things. Eating Halal is obligatory.
The opposite of 'Halal' is "Haram", which means 'Unlawful' or 'Prohibited". The other words used for Haram is 'Unacceptable' or 'Forbidden'. Haram foods and drinks are absolutely prohibited for consumption by Allah. Eating Haram is forbidden for every Muslim. If a Muslim consumes Haraam foods, he would be sinful.
The following products are definitely Halal:
Milk (from cows, sheeps, camels or goats).
Honey.
Fish.
Plants which are not intoxicant.
Fresh or naturally frozen vegetables.
Fresh or dried fruits.
Legumes like peanuts, cashew nuts, hazel nuts, walnuts, etc.
Grains such as wheat, rice, rye, barley, oat, etc.
Animals such as cows, sheep, goats, deer, moose, chickens, ducks, game birds, etc., are also Halal, but they must be Zabihah (slaughtered according to Islamic Rites) in order to be suitable for consumption. The procedure is as follows: the animal must be slaughtered by a Muslim by putting the animal down on the ground (or holding it if it is small) and slitting it's throat with a very sharp knife to make sure that the main blood vessels are cut. While cutting the throat of the animal without severing it, the person has to recite "Bismillah Allah-u-Akbar".
The following items have been categorically spelled out as being Haram:
Pig/Swine/Pork and its by-products.
Blood and and blood by-products.
Carnivorous animals.
Reptiles and insects.
Halal animals which are not slaughtered according to the Islamic Law.
Animals killed in the name of anyone other than God.
The bodies of dead animals or dead before slaughtering.
Alcohol and intoxicants such as Wine, Ethyl Alcohol, and Spirits etc.
The above mentioned items are Haram and should be strictly avoided by all Muslims.