Prophet Muhammad (Sallallaahu`Alaihi Wasallam) is reported to have said:

Halal is clear and Haram is clear; in between these two are certain things that are suspected. Many people may not know whether those items are Halal or Haram. Whosoever leaves them, he is innocent towards his religion and his conscience. He is, therefore, safe. Anyone who gets involved in any of these suspected items, he may fall into the unlawful and the prohibited. This case is similar to the one who wished to raise his animals too close to a restricted area, so that the animals may step into that area. Indeed for every landlord there is a restricted area. Indeed the restriction of Allah (Subhanahu Wa Ta'ala) are the Haram." (Sahih Bukhari & Sahih Muslim)

Glossary

Halal: Halal is a Quranic term which means allowed or lawful. Halal foods and drinks are permitted for consumption by Allah (Subhanahu Wa Ta'ala). Eating Halal is obligatory on every Muslim.

Haram: Haram is a Quranic term which means prohibited or unlawful. Haram foods and drinks are absolutely prohibited by Allah (Subhanahu Wa Ta'ala). Eating Haram is forbidden for every Muslim.

Mushbooh: Mushbooh is an Arabic term which means suspected. If one does not know the Halal or Harm status of a particular food or drink, such a food or drink is doubtful. A practicing Muslim prevents herself/himself from consuming doubtful things.

Halal Food Selection Guide

Instructions
      For a product to be Halal(lawful) for Muslim consumption all of its ingredients should be Halal. Following are some general guidelines with a partial list to keep in mind while doing shopping:

1. Read the ingredients on label carefully. Ingredients are listed according to the amounts present. The first ingredient listed is present in the largest amount.
2. Identify the Halal/Haram/Mushbooh status of each ingredient using this guide.
3. Search the ingredient name in the list and its status will be next to it.
4. If all ingredients of a product are Halal, the product is fit for Muslim consumption.
5. If any of the ingredient is Haram, the product should not be consumed.
6. If any ingredient falls under Mushbooh category avoid this product, as the Prophet Muhammad (Sallallaahu`Alaihi Wasallam) has advised us to avoid doubtful matters. This ingredient could come from a Haram source. To find out the status of this ingredient you should write or call the manufacturer of the product