I attempted to make this tutorial as simple as possible without getting too involved into the massive details of the HDR and HDRI. If you are the type of person that prefers a more technical approach, please refer to the reference links below. Also note that you are not limited to using 3 images for creating HDR’s.

->What is HDR/ HDRI?

Basically HDR refers to high dynamic range/ High dynamic range imaging; most commercial cameras can only capture images in a low dynamic range (LDR). By creating an HDR (in photography) we are combing several LDR images to create an image that expands the tonal range and detail of an image that a most cameras do not normally record.

By using a program like PhotoMatix we can create an HDR image by combining 3 or more LDR images with different exposure values.

->What is Exposure Value (EV)?

Exposure value is the value created between the lens aperture and the camera's shutter speed. It can also be used to measure the difference between multiple exposures i.e. if I shoot an object with an aperture value of 8 (f/8) and a shutter speed of 1/125. I can refer to this exposure as EV 0 (if properly exposed of course). To increase the EV of the next image to EV +1, I can change my camera settings to something like f/8 with a shutter speed of 1/60. By decreasing the shutter speed by one stop I am doubling the light coming into the camera and therefore increasing the EV from EV 0 to EV +1.
Typically when creating an HDR with 3 images, you want one of your images to be underexposed, one to be normally exposed and one to be over exposed based on the given scene and or object.
So if I decided to shoot 3 images with a one stop difference the EV's would be -1, 0, +1.
If I shot 3 images at one half a stop apart my EV's would be -1/2, 0, +1/2 and so on.

->Okay I have my 3 images how do I create the HDR?

To create an HDR image you have to use a program like PhotoMatix Pro or Photoshop CS2. I prefer to use PhotoMatix because it is very easy to use, requires only 3 images to create an HDR and does an excellent job of aligning your images together.

->To create an image in PhotoMatix Pro

1. Open all three of your images into PhotoMatix, you can do this by pressing ctrl+O, use the file menu or simply by dragging your images into the programs content window.
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2. Next press ctrl+G or navigate to the generate HDR from the HDRI menu.
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3. Next you should see a "Step One" menu confirming that the 3 images you have opened in PhotoMatix are going to create your HDR. Press OK
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4. After pressing ok the Exposure Value's menu will pop up and ask you to input your EV values for the images. Most of the time this will already be filled out correctly, if it is not simply type in the correct values from the three images into the input boxes next to the corresponding images. Press OK. (So if I had three images and they were exposed one stop apart I would give the first-overexposed- image a +1 the second image a 0 and the third -underexposed-image a -1)
Please note in this example the images I am using have EV's of 0, -1, -2
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5. Next the "Step two" dialog box will pop up Make sure that the standard response curve radio box is checked. Also be sure to check the "Align LDR images before generating HDR Image" , this will ensure that the three images will be lined up as correctly as possible.
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6. Poof, you now have HDR image being displayed on the screen (as best as it can). Now we want to tone map this image so that it can be viewed and printed normally using standard equipment. From the HDRI menu select Tone Mapping. On this menu you will be able to adjust the saturation, white clip, black clip, etc. Adjust these settings based on your own subjective needs. Press Okay, then ctrl + S to save you HDR image.
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Your viewable HDR image is now created and can be opened in your favorite photo editor program to make any final corrections needed.

>>>Tips for successful HDR Images<<<        

->Use steady support

This is imperative to create an HDR image that is aligned properly. It will not only save you time but it will also create a sharper HDR image for you. Use a good quality tripod or other support such as a red pod or a solid bean bag.

->Set your camera's AEB

If your camera has this ability (Auto Exposure Bracketing), this will save you a great deal of time in calculating your EV's manually.

->Use a remote Shutter Release

By using a remote shutter release you greatly reduce the chances of getting blurry images from camera shake. To take this one step further use the mirror lockup function on your camera -if available.  

->Stay away from long Exposures

Long exposures definitely have there place in Photography but not so much in creating HDR's. Having tried this before I can definitely say that the images are very difficult to line up and any digital noise that you have in the photos will be greatly amplified from the increased contrast added during the tone mapping process. This does not mean it is impossible, just difficult.

->Use Fast shutter Speeds for movement-

It is not recommended to create an HDR image where there is a lot of movement involved. Although is some instances you can achieve this, you may have to line your images up in a program like Photoshop before combining them into your final HDR.

->Shoot in the RAW

To achieve the best results shoot your images in Camera Raw format and then convert them into 16 bit tiffs.

References:

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> A tutorial for Photoshop Users

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