Bird Feeders
Bird feeders come in seemingly endless varieties.  In fact, however, almost all of them are variations on
a few basic designs.  
Tray or Platform Feeder
As the name implies, a tray feeder is simply an open tray that holds
birdseed.  An outdoor table or stool or even a flat rock could serve as
a makeshift feeding tray, but specially made trays are better.  A tray
feeder often has raised sides to keep seed in.  Some have screen
bottoms for drainage; if not, they should at least have drain holes
drilled.  Some are equipped with roofs or covers to keep seed dry.

Tray feeders can be placed on the ground or hung above ground.  
Some are specifically equipped for one or the other, with legs for
ground or a wire or chain for hanging.

A tray feeder is the best type to use for feeding millet and cracked
corn to ground-feeding birds such as sparrows, juncos, and towhees.  
It can also be used for meal worms or fruit.

The major disadvantages of tray feeders are the relatively small
amount of seed they hold, and the fact that it is exposed and easy
picking for squirrels and nuisance birds.  Of course, if you are
intentionally feeding squirrels and jays, tray feeders work wonderfully.
Tube Feeder
A tube feeder is a vertical tube, most often made of plastic or glass,
with several openings, or feeding ports, where birds can perch and
eat.  The shape of the tube may vary, but the idea is the same.
Tube feeders are ideal for feeding black oil sunflower seeds or
sunflower chips to chickadees, finches, grosbeaks, nuthatches, and
other birds that like to feed above the ground.  Some have very small
openings for feeding nyjer thistle seed.  

Seed in a tube feeder is fairly well protected from the elements.  Tube
feeders are available in basic models, or with extra features such as
adjustable feeding ports for sunflower or nyjer seed, adjustable
perches to keep larger birds from landing, two or more tubes on a
single feeder for feeding different kinds of seed and serving more
birds at once, and squirrel-blocking cages that cover the feeding ports
when a heavier animal tries to eat from the feeder.
Hopper Feeder
A hopper feeder consists of a bin, or hopper, that holds seed and a
tray or trough.  Seed pours into the tray or trough through openings at
the bottom of the hopper.  As seed is consumed, gravity keeps seed
flowing into the trough until the hopper is empty.  

Hopper feeders are well suited for feeding sunflower seeds and most
seed mixes.  The hopper part of the feeder holds much more seed
than a simple tray, and keeps it somewhat sheltered from rain and
dampness.  

Hopper feeders vary widely in appearance.  Often they're made in the
shape of barns or houses.  Sometimes they look a lot like tube
feeders.  If it has a tray or trough at the bottom where seed comes out,
it's a hopper.
Screen Peanut/Sunflower Feeder
Sometimes similar in shape to a basic tube feeder, this type of feeder
has walls made of wire mesh just large enough for sunflower seeds or
shelled peanuts to pass through.  

A peanut feeder offers very little protection from the elements.  It does
allow birds to get seed from anywhere along the mesh that makes up
the feeder walls, so it can potentially accommodate a lot of birds at
once.  Some all-metal models resist destruction by squirrels as well.
Thistle Feeder
Designed especially for feeding tiny nyjer thistle seeds to finches,
thistle feeders are usually one of three types.  It can be a tube feeder
with narrow slits instead of wide feeding ports.  It can be a screen
feeder, similar to the peanut/sunflower feeder, but made with a much
finer mesh.  Or it can be a simple mesh bag or "sock."

Thistle feeders also are appropriate for feeding fine sunflower chips,
or a mixture of nyjer and fine sunflower chips.
Nectar Feeder
Naturally, these feeders are made for nectar or sugar water.  In our
part of the country, they cater to hummingbirds.  (Elsewhere, they're
also made for orioles.)  

Nectar feeders come in a huge variety of shapes and sizes, from basic
and functional to highly ornate.  Some are equipped with guards to
keep out bees, wasps, and ants, which are also attracted to the sweet
liquid inside.  If these insects are a problem for you, consider trying a
feeder with white guards and accents instead of yellow, which studies
have shown actually attracts bees and wasps.  
tube feeder