The arabic word  (Halal) means lawful. In the Holy Quran, Allah commands Muslims and all of mankind to eat of the Halal things. Among the many verses of the quran that convey this message, here are a few:

O mankind! Eat of that which is lawful and wholesome
in the earth, and follow not the footsteps of the devil. 
Lo! he is an open enemy for you. (2:168) 
 

O ye who believe! Eat of the good things wherewith 
We have provided you, and render thanks to Allah
if it is (indeed) He Whom ye worship. (2:172)
 

Eat of that which Allah hath bestowed on you as food 
lawful and good, and keep your duty to Allah in Whom
ye are believers. (5:88)
 

The following products are definitely Halal: 

1. Milk (from cows, sheep, camels, and goats) 
2. Honey 
3. Fish 
4. Plants which are not intoxicant 
5. Fresh or naturally frozen vegetables 
6. Fresh or dried fruits 
7. Legumes and nuts like peanuts, cashew nuts, hazel nuts, walnuts, etc. 
8. 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. The animal should be put down on the ground (or held it if it is small) and its throat should be slit with a very sharp knife to make sure that the 3 main blood vessels are cut. While cutting the throat of the animal (without severing it), the person must pronounce the name of Allah or recite a blessing which contains the name of Allah, such as "Bismillah Allah-u-Akbar".