There are two types of 3D TVs on the market today: active and passive. Both will work with any source--there's no such thing as "active" or "passive" 3D Blu-ray players, for example--but they produce the 3D effect in distinctly different ways.
Active 3D was first widely introduced in 2010 by most TV makers, while passive 3D widely debuted in 2011. The main difference is in the glasses: active glasses use liquid crystal shutters that run on batteries, while passive glasses use simple polarizing lenses, similar to what you'll get in most U.S. 3D theaters. You'll hear a lot of claims about each if you're comparing 3D-compatible TVs, so here's what you need to know.
3D technology is directly tied to brand this year, so if you're buying a 2011 LG or Vizio 3D model, it will be passive. The exception is LG's 2011 3D plasma TVs which, like all 3D plasmas, use active glasses. While Vizio produces the widest range of 3D TVs, starting with its 32-inch E3D320VX ($549), LG is the main developer of the "pattern retarder" technology used by all current passive sets. Toshiba, alone among U.S. TV makers, has both an active and a passive 2011 3D TV.
The main market advantage of passive 3D TV is inexpensive glasses. Most LG and Vizio TVs come with two to four pairs of passive glasses. Very few active 3D TVs include glasses by default, although throw-ins and discounted bundles including glasses are common.
Bought individually, active 3D glasses range from $50-$130 each, and one brand's active glasses won't work on another brand's TV. Extra passive glasses cost $10-$20 each from TV makers, and you can use pretty much any circular polarized passive glasses--including off-brand versions or even ones "borrowed" from a theater, with any passive 3D TV.
Since they don't have electronics or batteries, passive glasses are lighter and more comfortable than most active glasses--although 2011 active glasses are generally lighter than their 2010 predecessors, and we've found most of them comfortable enough. Passive glasses come in many form factors, including designer and clip-on versions for people who wear regular glasses. They also don't introduce flicker when you're multitasking with a laptop, phone, or other screen while wearing them; nor under bright fluorescent lighting.
Active glasses flicker in both circumstances, but in our experience they don't usually introduce visible flicker when you're actually watching 3D TV. They do need to be turned on and synced with the TV, although that's usually a simple process. Their batteries also need to be periodically replaced or recharged.
In our experience 3D picture quality varies greatly depending on manufacturer, model, glasses, technology type (plasma, LCD or LED) and even screen size. That said, we can make some broad generalizations between active and passive based on what we've seen.
Passive 3D TVs have a brighter 3D image than active, although active 3D TVs can get bright enough for most viewing environments. Passive generally causes less crosstalk--a major 3D-specific artifact--than active on many models, although other active models are just as good as passive TVs in this department.
Active 3D TVs don't show jagged-edge artifacts and line structure that can be seen on passive models, although these artifacts are less visible on smaller screens and farther distances. They also keep their 3D effect better when seen from extreme angles to either side or above and below the image--although from most normal viewing angles, passive 3D TVs have no issues maintaining the 3D illusion. The fact that passive isn't available on plasma TVs is also an issue to home theater enthusiasts who don't want to buy an LCD.
Our main hang-up with passive 3D TV is the presence of the artifacts mentioned above, which we find especially distracting at the closer seating distances and with the large screen sizes favored by home theater enthusiasts. But with the practical and certain picture quality strengths of passive 3D, especially in bright rooms, an argument can certainly be made that it's the better choice overall.