What is an evaporator coil?

Layman's description of an evaporator coil.

The coils basic design is something like a radiator on a car. There are two flat radiators leaning against each other in an "A" shape. It is referred to as an "A" coil. It's got tubes that are surrounded by aluminum fins. It sits inside a sheet metal box (coil enclosure) and the whole package is usually installed on the supply (or output) side of your furnace (most of the time it's inside the box that sits on top of your furnace).

The coil is connected to the outdoor condensing unit by two copper refrigerant lines. One of these copper lines delivers refrigerant to the coil, and one of them takes it back to the condensing unit. When everything is operating like it should in the summer, that refrigerant goes through a number of transformations and makes the evaporator coil very cold. When the blower moves air across the coil, cool air is distributed through your duct system.

Because the cold coil has ambient warm air blowing across it, it creates quite a bit of condensation. That condensation must be removed or else you're going to have water everywhere. The coil itself has a drain pan which is usually connected to a hose that leads to a floor drain in the basement.