While the term may seem generic, steel wheel generally is used to refer to the most basic type of modern automobile wheel, made from stamped steel.
Typically speaking, these wheels are made of three to four pieces of steel stamped into shape and then welded together. They generally consist of two plates which make up the vertical portion of the wheel which is mounted to the car via the lug nuts, and a third rolled and stamped piece which forms the rim of the wheel. Older steel wheels were often riveted together, but in modern times various low-cost welding processes make that impractical, and it also reduces the overall strength of the wheel.
Steel wheels are sturdy and fail gracefully, which is to say they tend to bend rather than break. Mag wheels are more likely to simply break, though it is fairly rare. Unfortunately they are also very heavy, and are often half again to twice as heavy as their cast counterparts.
The upgraded version of the steel wheel is the rally wheel.