There are two basic types of studio flash systems: two-piece (consisting of a power pack and one or more flash heads) and monolight (in which the flash head and power pack are combined in one unit). The traditional separate power pack system generally offers more power and quicker recycling, and you can plug several flash heads in a single power pack. Monolights, being self-contained, are easier to position just about anywhere and to transport, and cost less. There are power-pack systems that can run on battery power, and many monolights offer this capability, so both types of systems can be taken into the field when necessary, but a monolight is more compact.
A monolight is lighter and takes up less space than a separate flash head and power pack. This monolight advantage lessens as the number of heads increases, however: Since each monolight contains both a flash head and power pack, the monolight weighs more than a pack-and-head system flash head alone. This is mainly a consideration when hanging the head on a boom.
With a pack-and-head system, the power pack plugs into the wall plug via a cord, and each head plugs into the power pack via a cord. This puts a lot of cords on the floor, stretching around the studio from the power pack to the wall outlet and from the power pack to each flash head. With a monolight, the unit plugs into the wall plug, and that’s it. Each monolight can be plugged into the nearest wall plug, thus minimizing the cords-everywhere problem. But you need a socket for each monolight you want to use.
Of course, if your system—pack-and-heads or monolights—offers wireless control, then you just need one wall plug for either system. And if your system provides battery operation, you don’t even need that.