I will try and summarize what a tripod is, and hope you follow the links given above by Neuro and these here for information on selecting tripods.http://blog.reallyrightstuff.com/choosing-a-tripod-part-1/
A professional level tripod consists of 3 parts. The legs, the head, and QR (quick release) plate clamp. An amateur tripod is an all in one, usually doesn't have a QR plate clamp but some do.
A professional level tripod may come as a kit - but usually you will buy the head and legs separately. The QR plate type will be determined by the head you choose.
So you will decide on legs first. They come in a variety of materials but the most common are aluminum and carbon fiber. Aluminum is generally cheaper, heavier but can be a good starting point. If your hiking or traveling, carbon fiber may be the best to start with and has other benefits such as stability in wind and vibration reduction. Size is also a consideration, both collapsed and extended. How tall are you? You don't want to be forced into crouching to look through your viewfinder. Will you be traveling and need to carry it on an aircraft? The best advice is to buy the best you can afford now, that way you don't keep spending in the future for something that you could have purchased once.
The head is next and the most common for photographer is the ball head or BH. Generally these have three controls, one for panning, one to release the ball to manipulate your camera and a third to adjust the tension. There are many particulars to the different manufacturers models but in general, its a ball within a clamp and a stem leading up to the QR clamp. They can be light weight and small for use on a monopod or very heavy and stable with even the heaviest load - and cheap to very expensive. You need to decide what weight spec's you need for your camera, lenses, accessories and future needs - this applies to the legs too.
The Quick Release Clamp and Plate will be decided by the head you chose. You should look for Arca-Swiss compatibility. Another type is Q2 from Manfrotto and they also have other forms of the QR system as they can't seem to make up their minds - and they are proprietary while Arca-Swiss is an open defacto-standard.
Why do you need a QR system? Because it is a HUGE
hassle to unscrew the camera or lens off of the tripod. Some lenses have tripod collars and these too will need plates. The plate is the part that is attached to the camera and lenses and they match the QR Clamp type (Arca-Swiss). With Arca-Swiss there is generally a knob that is turned to clamp onto the plate and it keeps your equipment from an unexpected disaster. Some of these come with safety mechanisms that keep your camera with plate from sliding off when your loosening the clamp.
As you may have noticed, tripods can be very expensive or down right cheap. I like my first tripod, except for the Q2 QR system and wish I had gone Arca-Swiss. My investment was $160 for the legs and $160 for the BH. And now I'm spending money to convert it to Arca-Swiss and even more money because I want to put a heavier load on it with a Panoramic setup and better leveling. So the $375 I've spent could have gone nicely towards an even bigger expense now. But, if your willing to pay to change in the future, you can spend a bit less now and learn my lesson that hard way too.
I kind of mentioned it, but while looking at legs and the head take note of the max weight they can handle. You don't want to cut it too close. And that is an indication of higher quality and stability. You don't want your legs flexing from the weight or the BH (ball head) giving away and allowing your camera to droop.