In the beginning, all LCD's were matte. A matte screen means that it's not shiny. You really can't catch a reflection off of it. Matte screens are great, you can use them in a well lit room without having to deal with reflections.
Then Apple decided that matte screens are ok but glossy screens are great. Glossy screens have shiny glass over them. They make darks look darker, giving an illusion of deeper contrast and sharper colors. And the trade-off is that they are like mirrors. They reflect everything. In a well-lit room, it's like looking in a mirror. Outdoors, all you see is what's behind you. Unless you do all of your computer using in a Cave, glossy screens are not good.
Then Apple made a horrible decision. Glossy screens are so great, we don't need an option anymore. Everything will be glossy. The glossy screens on Aluminum iMacs are horrible. But the iMac's are the only real choice for their price/performance bracket, so I have to recommend them anyway. And they're otherwise great computers, you're just going to have to deal with tons of really bad glare while you use it.
Apple thankfully brought back the matte option on the new MacBook Pro 17". So the current screen situation is this: iMac 20" and iMac 24" are all glossy. MacBook 13" MacBook Air, and MacBook Pro 15" are all glossy. The MacBook Pro 17" is glossy but has an available $50 matte option. If you're spending all that money on the laptop, I highly recommend you spend the extra $50 and get the screen you can actually see.
Of course the Mac mini and Mac Pro don't have built in displays. Apple makes stand alone displays, which used to be all matte, but recently Apple again replaced the 23" cinema display with an all new 24" display that is glossy. So the 20" and 30" cinema displays are matte, but the 24 is glossy.
Personally I would love to replace my Mac Pro tower with a high end iMac in 6 months to a year. But the glossy screen is a deal breaker. It's just unbearable to constantly be fighting the reflections.
Hopefully, Apple well get the hint and make matte screens an option on all their LCDs. The photo is of a new Aluminum MacBook 13" and a 20" iMac, both glossy. You see what's behind you as well as you see what's on the screen.