Distortion with ultra wide lenses?

I went to Best buy and bought a Tamron 10 to 24 ultra wide for my D7100, a more cost effective alternative to the Nikon version i was looking at
anyway, i like the pics i take with it although i do notice some distortion, like "stretching" at the widest angles
check out the attached pic as an example, the outer parts of the pic seem stretched in a way
its not a biggie to me, i kinda like the different way it makes the pics look
but i was wondering if this was common with ultra wide angle lenses?

What you are calling distortion (stretching) is not optical distortion; it is a natural result of an error in perspective. For most applications, the photographer and the observer of his/her work is not concerned if images do not display faultless geometrically correct perspective.

Allow me to explain: If you stand before a glass window and gaze out at a sprawling vista, you can trace out on the glass with wax pencil, the outlines of objects. This drawing is geometrically correct. We can say the drawing depicts the "human perspective".

We can duplicate this perspective with a camera. The scene is photographed. The camera lens need not be any particular focal length. We now view the image just recorded. To view with the proper "human perspective", our eye must view the image from about the same distance away as the focal length of the lens that took the picture. Now this is easy to do if the camera is gigantic and loaded with a huge piece of film. Such a camera will sport a long focal length lens, and we can easily see the image from a distance about equal to the focal length.

Nowadays, our cameras have shrunken to miniature and sub-miniature. We mount with super short lenses. It is impossible to comfortably view an image from 10 or even 24mm away from the eye. Additionally the tiny images are useless unless enlarged. So we view a print or computer monitor with about 10X or 12X magnification applied.

Factoid: To view an image and cause it to appear undistorted our eye must be at focal length distance multiplied by the magnification. So you mount a 10mm and view a print or monitor image magnified 12X. Your eye must be 10 x 12 = 120mm from the displayed work. That's 4 inches. You will need a magnifying glass (reading glass) to allow you to focus your eyes at this close range. If you don't follow these modest laws of physics, your image will appear distorted. Follow these guidelines and the distortion will evaporate.

I know this stuff is not easy to grasp; you should take the time and study perspective. Because this is all offbeat information to most, I classify it as gobbledygook.

The stretching you might be referring to is perspective distortion. It's all about distance ratios. Take a picture of a dogs face straight on, about 3 or 4 inches from the dog's nose. That's a good example of perspective distortion. Wide angle lenses will also tend to curve straight edges to some degree.

Yes, of course it is common - the distortion occurs between those parts that are both closest and furthest and center and edges of lens and amount of bending needed to form image. By far the "worst" when there's a closer object, like a nose, in the middle of the picture and closest.

Yes, that's what happens with very wide angle lenses.