We all implicitly know what texture is. For example, some people like pulp in their orange juice, some like no pulp; some prefer a high thread count for their pillow cases for a smooth, satiny feeling, while others like a courser fabric; some people prefer smooth peanut butter, while those of us who aren't deranged and like to keep it 
prefer chunky.
Most of us can certainly feel texture on our body parts, but we have also learned to associate these feelings with our visual recognition of different textures. Therefore, we can see a tree trunk's texture and know ahead of time whether the bark is smooth or rough. We can see a sweater's texture and know that it will probably be soft. This is learned with experience, of course, but it means that texture is a visual element as well as physical property.
In art and design, texture is almost entirely virtual and/or invented, as opposed to real. A painting of a tree can simulate the texture of that tree with variations in light and dark (i.e. value). A 3D animation can simulate the texture skin of a dinosaur's skin, a simulation that is likewise invented, as we've never really seen actual dinosaur skin (I hope). The examples above are all images of real textures, and are thus virtual or simulated.
Texture as an element can add the properties of dimension, age, slickness, cost, speed, or even an element of danger to a form. Below is a gallery of different implied textures that illicit very different feelings.
