There are really 2 basic approaches to keeping squirrels out of your bird feeder. The first is to simply try to block them from getting to the feeder. There are two common types of feeders that do that.
Baffled Bird Feeder – By simply restricting access to the the food in the feeder, you frustrate them and they move off. The advantage to using a baffle is that it more or less separates the squirrel proof barrier from the feeder itself, so that the bird have free access to the bird seed, and any size bird can still use the feeder. This is often the more aesthetic approach. This is often done by pole mounting the feeder, and putting the baffle on the pole. Other types use a hanging arrangement and the baffle forms a roof over the feeder.
The disadvantage of this approach is that squirrels can leap large distances, and are incredibly agile climbers, so that they will often find a way to land on the actual feeder and bypass the baffling. With the top mounted feeder you may also find a squirrel smart enough to climb down and cause the feeder to swing, and while they may not gain access to the feeder itself they can cause the bird food to spill to the ground, where they can get all the food they want.
Caged Bird Feeders – Another very common solution is to use a caged feeder. These are easy to spot, the construction includes some type of cage around the actual feeder area. It’s a fairly open mesh, and in fact a tube style caged feeder looks similar to an electronic bug zapper with the open mesh surrounding the tube.
Caged feeders are better at keeping the squirrels out of the feeder, but they can still get to it and scare off the birds that are trying to use the feeder. This is often the case when you are trying to attract smaller birds.
The second approach to getting rid of squirrels in your bird feeder is to move them out. The much more effective types of squirrel proof feeders are those that cause the squirrel to fall off or be thrown from the feeder. There are two types of those feeders as well, weight activated feeders and the battery powered or electronic motorized feeders. While this won’t harm the squirrels they are fun to watch in action.