Cameras can be held in two positions: horizontal or vertical (also knows as landscape and portrait). Photos taken in the vertical position are later on rotated 90 degrees (either clockwise or counter clockwise). Rotating the photos can be automated if your camera sports an orientation sensor.
If you just take a few photos every now and then rotating them manually is not a big deal. Usually after downloading the photos to your computer you will browse through them and when viewing a photo that needs to be rotated you will fix it using your software rotate feature. However when taking hundreds or thousands of photos this process can become tedious and long. The good news is that this process can be automated if the camera you are using includes an orientation sensor.
An orientation sensor is an electronic device built into the camera that can sense if the camera is held horizontally or vertically and if the camera is flipped or not (there are two vertical positions also known as 90 degrees or 270 degrees relative to the horizontal position). We will not get into the details of how this sensor works (there are a few options utilizing some physical feature such as gravitation or magnetic fields).
When taking a photo the camera saves the orientation sensor reading as EXIT data included in the photo file. EXIF stands for Exchangeable Image File Format and it defines a set of tags and semantics that can be added to existing formats such as JPG and TIFF to provide more information about the photo. One such piece of information is the camera orientation.
Software can later on use the EXIF orientation data to automatically rotate photos as needed. There are some options for how you can utilize this orientation data for your benefits, here are a few:

Camera built-in rotation software: Some cameras include an automatic mode in which the camera rotates the photo based on the orientation reading immediately after the photo was taken using build-in rotation software. Using this mode the photos saved on the camera memory are already rotated in the right direction and no further processing is needed.

Photo download software rotation: Some software can automatically rotate photos as they are downloaded from the digital camera to the computer hard disk. Before saving a photo to the hard disk the software checks its orientation and rotates it as needed.

Batch photo processing: Software that perform batch operations on photos. Usually such software can do many common photo processing operations such as contrast enhancements, re-sampling, red eye removal and more. They can also rotate photos based on the EXIF orientation data.

Photo viewers: Some photo viewers can utilize the EXIF orientation data when displaying photos. As opposed to the other solutions in this case the photo file is never changed instead every time a photo is displayed on the computer screen the displayed image is rotated as needed based the photo EXIF orientation data. Some professionals prefer this method as it solves the need to rotate photos in order to view them while it does not change the original file. Rotating a photo can degrade its quality if not done properly and thus leaving the original file as-is and only rotating the viewed image is a good way to guarantee no quality loss.

Orientation sensors are a nice addition to the ease of digital photography. If you plan to take many photos and you want to save time by automatically fixing their orientation when downloading them to your computer check if your camera includes an orientation sensor and if it does not make sure that your next camera does include one. Today such sensors are usually included in more expensive cameras as orientation sensors have not become a standard feature in all digital cameras ? at least not yet