Steam tables are designed to hold hot food items for a long period of time. They feature several different heated wells into which water is placed for a pan of food to float in. The steam created in concert with the hot water help maintain an even temperature on the pan as well as keep the food moist until served. Each well of a steam table is designed to fit both water and a full-size pan of food. One some models a separate, larger pan is required to hold the water as well. The only difference is what type of heating element the table has. Some elements are able to be submerged, others are not. Be sure to check whether an extra pan is required or not because if you dump water into a well whose element is not rated for wet use, the element will have to be replaced. The same goes for dry firing an element that is meant to operate under water.
The power source itself is something else to consider. Kitchens that already have or plan to have gas-powered equipment will want a steam table that runs off of gas, but you will want to make sure the inlet size is compatible with the main line. For electric units, different voltage options are available if the standard is not suitable for your current hookup.