Strictly speaking ‘halal’ means permissible. In most common use it is applied to dietary requirements of Muslims. While all vegetarian food is permitted, there are some restrictions on non-vegetarian food. Also the animals permitted to be eaten are to be slaughtered as per religious requirements. Generally it means to slaughter in a way so that most of the animal blood gets out of the body and to do it in the name of God with the intention of taking His permission.
Any animal by-product which is used in other food items should also be through a halal process. This results in dietary requirements that restricts the use of gelatin, alcohol, vanilla extracts, etc. The halal shops cater to these needs.
But it is not only this. A typical halal shop is a full blown ethnic grocery store. You can get anything from flour, bread, rice, spices, ice-creams, yogurt, milk, and many many things more.
Saad Meats & Bakery is a well known establishment in the Detroit suburbs. They have a constant stream of customers throughout the day which includes not only Muslims but the wider community.