What's a Seamless Texture?
A seamless texture is an image that can be placed side-by-side with itself without creating a noticeable boundary between two copies of the image. Consider the following seamless texture.
If the seamless texture were repeated four times, it would create a continuous pattern without it being obvious where one copy of the image began and another one ended. This is demonstrated below.
By contrast, consider the following texture photo that the seamless texture had been based on.
When repeated four times, visible seams form at the image boundaries. These seams mean the image is not seamless.
How Are Seamless Textures Made?
Seamless textures can be made with a variety of software. Many artists use generalist applications such as Photoshop, where seams can be carefully painted away. Others prefer applications like Genetica which are specifically created for seamless texture artists and automate much of the process. A tutorial for creating seamless textures with Genetica can be found here.
Why do People Care?
Seamless textures are especially important in 3D computer artwork, where arbitrarily large surfaces can be covered by a seamless texture without the appearance of distracting seams. The texture presented at the beginning of this article, for example, could be turned into a large floor.