Others have already described pattern flying and pointed you to some good resources like the NSRCA website, so I'll comment on pattern ships in very general terms.

Pattern planes typically feature longer fuselages in proportion to their wingspan than sport or 3D aircraft. They are designed to fly straight in a "hands off, go where you point them" kind of way, and to fly inverted as easily as they fly right side up.

Most pattern planes need to be landed faster than a typical 3D or Sport airframe, but they don't generally exhibit the same same tip stall characteristics that scale aerobatic aircraft or scale warbirds normally do.

Knife edge flight is normally very good with modern pattern airframes, and there are some pattern/3D hybrid airframes available that try to do both kinds of flying well.

The general contradiction between these two flying styles, 3D and pattern, is in the execution of the manuvers. Pattern planes are normally not set up with much throw on the control surfaces. Pattern pilots want nice, smooth, easily controlled manuevering. 3D flying, on the other hand, requires a lot of control throw. 3D pilots need a lot of throw to keep the plane pointed up while hovering, and a lot of the nose-up (high-alpha) manuvers that are done in 3D flying are done at slower speeds.

Basically, if you take a hybrid 3D and pattern airframe and fly it, it will fly pattern well but not 3D if the throws are gentle. If you set the throws up with greater range of movement, then the plane will fly 3D well but probably not pattern, it would become fairly "twitchy" and too easy to overcontrol and fly smoothly.

There is a misconception that pattern style flying is only done competitively, and that you need an expensive, 2-meter composite airframe to do it. General pattern flying is done by a lot of sport pilots who simply want to learn aerobatics and improve the skill and smoothness of their flying. A lot of very popular ARFs and kits make good "pattern trainers":

Goldberg Tiger 2/Tiger 60 ARF or Kit
Tower Hobbies Kaos .40 ARF
Great Planes Venus .40/ Venus II ARF
Phoenix Models SeaBee
World Models Zen/Groovy/Tai Jai/Aeropet/Intruder ARFs in various sizes
CMPro Magpie/Quest/Leo/Swallow ARFs in various sizes
Black Horse Models Bravo 303 MkII/Home Run ARFs
Thunder Tiger Imagine 50 ARF
Sig Kobra/Kougar MKII/King Kobra kits

...and many more.

Pattern flying is more about disciplined, precise, smooth flying than having a particular airplane. It's very graceful and enjoyable to watch.

Some of the 3D/Pattern style airframes that will fly either well depending on overall power/throw setups are:

Great Planes Reactor .50 ARF
Lanier Razor 3D ARF
Model Tech Twister 3D ARF
Hangar 9 Funtana 50/100 ARFs

A few of the scale aerobatic planes can be flown as pattern planes:

Extra 300S (take your pick)
Staudacher 300
Giles 202

Again, the general characteristics are simple smooth, stable flying and throws set in a way that will give good manuverability without a tendancy to snap roll too easily. There's no reason why you couldn't fly pattern with a Bipelane that happens to feature a longer than typical fuselage, the Great Planes Super Skybolt ARF (for example) could be set up to fly the Sportsman's Pattern schedule smoothly and easily.

Hope this explanes a little better just what the heck a "pattern plane" is. Hopefully we'll get some additional thoughts and feedback from others on this topic.