Just condensing some of the ideas others have expressed into a simple question and answer format. If anyone wishes to use it they may, no credit needed. Also feel free to edit where needed, I'm not sure if I got some of the terminology just right.
What is FTA?
Free-To-Air or FTA as it is commonly known, is the reception of unencrypted satellite broadcasts.
What is the difference between an encrypted and unencrypted broadcast?
Encrypted broadcasts utilize equipment that allows the signal to be scrambled and therefore only viewable by equipment that is authorized to receive the broadcast. Encryption is applied to broadcasts by both pay satellite services and commercial broadcasters.
Unencrypted broadcasts do not use equipment to scramble their broadcast. Therefore any receiver that is able to utilize the broadcast format will be able to view the broadcast.
Isn't getting pay satellite service FTA?
No. Pay satellite services are encrypted and only intended for authorized customers. The act of loading third party software on a FTA receiver in order to decrypt these encrypted signals is called satellite piracy. Satellite piracy is illegal.
What types of channels are available on FTA? / Can I get HD channels?
In North America, most FTA channels are either religious or in languages other than English. There are channels that do not fall into either of these categories, but these two categories constitute most of what you will find unencrypted.
While there are a handful of channels available in HD most channels on FTA are still broadcast in SD.
Why would I be interested in FTA if I can't get any premium channels?
Since North America, unlike other parts of the world, has only a limited selection of programming available on FTA, many view FTA as more of a hobby than as a substitute for pay television.
As a hobby, FTA is very hands on and can provide a sense of accomplishment from getting your satellite equipment setup and working. After your equipment is up and running, you can enjoy the thrill of hunting for and finding new channels. Sometimes what you find up there will surprise you. As you gain experience and become more familiar with your satellite equipment, you will discover which satellites typically have the channels and feeds you are looking for.