Martha asked about folding kayaks and Sheryl about single VS double kayaks.
I have had a bunch of kayaks- plastic, fiberglass, wood and folding. I currently have a Folbot single and double.
http://www.folbot.com, (and a wood Pygmy single
http://www.pygmyboats.com) One of the best sites for information is:
http://foldingkayaks.org/ And there is a book by Ralph Diaz The Complete Folding Kayaker. Have a look at these and feel free to pmail me a message if you want my take on anything. Folding kayaks are great, they are stable, very seaworthy, can be sailed and ar, of course, poatable. One made it accross the Atlantic. They are the boat of choice for Arctic expeditions and military commandos so don't worry that they are made out of fabric, they are tough.
As to the single VS double-this in one of the most classic questions in all of kayaking. There is no one right answer. I prefer paddling a double with my wife. She prefers two singles-thank you. I go out with my younger son and the double is great. When my older boy is paddling I cannot keep up with him. The advantage of a double is that one person can take a break, fish, look at birds with binoculars, rest, get food and any number of things. A double carries a lot, ours will carry 600 lbs of people and stuff, somewhat like a canoe. A double will tend to sail better than a single. A double is more stable than a single. A double is faster if paddled by both people, slower when paddled by one.
A single is much easier to roll than a double, almost impossible to roll a double. A single needs a rudder much less than a double. It is sleeker. But most of all, you are on your own which is why a lot of people go out, to get some alone time. A single is much better if you are going to surf waves or go into caves.
If I were starting out and intended to paddle a lot with another person I would start out with a double and then add a single. Now you can go out with one, two or three people. And you can have one in a single and one in a double.
I could go on but I'll just wait and see if there are any specific questions.