There are basically five kinds of bird feeders, differentiated by the type of seed they can accommodate and the types of birds they appeal to. Tube feeders with large holes are made to be filled with sunflower seed to attract birds like cardinals, chickadees, finches and jays. Tube feeders with smaller holes are good for thistle seed, favored by goldfinches and other small birds. Most squirrel-proof bird feeders are tube-type feeders; because these use small perches for birds, manufacturers can adapt them easily to thwart squirrels. Most of these feeders are hung from a tree branch or eave.
Enclosed hopper feeders sometimes resemble little barns, or sometimes are cylindrical. They hold a lot of seed and appeal to a wide variety of birds, but standard models aren't very squirrel-proof. This type of feeder can be mounted on a pole as well as hung from a tree branch, but most aren't very weatherproof -- a problem if the bird seed gets too wet and moldy.
Open tray or platform bird feeders are the simplest type of bird feeder. These are pretty much what they sound like -- an open tray of food mounted on a deck railing or on a pole. The upside is that any bird has access to the feed. The downside is that squirrels and pretty much anyone else can also eat the seed. This type of feeder isn't protected from weather, so drainage holes are needed.
Window feeders are interesting -- these attach, usually with a spring-loaded pole, to your window casing so you can fill the feeder from the inside of the house. Most have some kind of one-way mirror so you can see the birds but they can't see you. Different feeders appeal to different birds. We found squirrel-proof versions of every type except window feeders.
Squirrel-proof feeders usually work in one of three ways, according to Bird Watcher's Digest. Some keep squirrels off the feeder entirely, using a baffle so that squirrels can't get a good grip on the perch. Others deny squirrels access to food, using a cage or weight-activated mechanism. The last type actually boots them off, using a device that repels or ejects squirrels without harming them.