Seed spreaders come in three basic styles, Hand-Held, Push, and Pull-Behind. They vary greatly in price and efficiency, and each fills a specific need. Choosing the one that suits your needs is not difficult at all. It all boils down to how much seed/fertilizer you are going to be spreading.
Hand-held will sling seed and/or fertilizer quite a distance. Some are designed with larger hoppers, and have a shoulder strap to help distribute the extra weight. If you only have a small project like re-seeding bare spots, or if your overall lawn is small, this may be all you need in a spreader.
Push spreaders have larger capacity hoppers than the hand-held, and handlebars to push them with. The broadcast mechanism is activated by the rolling of the wheels. Prices vary according to quality and size. These will handle small to moderate size jobs.
Pull-behind spreaders are designed to cover a lot of ground, and do it in an efficient and timely manner. They have larger hoppers than the other models, and a hitch to connect them to your lawnmower or 4-wheeler.