All the leaves on a tree are the same, aren't they? Well, they're the same shape, the same colour, the same texture, so they must be the same. No! This is a scan (not a photograph) of five leaves from one clump of leaves on one branch of a bush.
They all have the same general shape. If you were an expert, you'd know which bush they came from (a camellia). They maintain their 'same' general shape but vary in size. They're all green, but have subtly different shades of green. They all have veins which seem to trace the same pattern, but those patterns are very slightly different. Now, let's go to a close-up of another type of leaf.
All around you, in the natural world, you'll find repeating patterns. They look the same, but are actually irregular. At first, they might seem to be identical. On closer inspection, you find that they differ, and each one has slight mutations within it. The basic shape might be similar, but the size changes. The patterns on the surface also change.
For example, all the leaves on one bush look alike. They might be different sizes, but they appear to be the same general shape and to have the same patterns, more or less. Here's a scan of another ordinary everyday leaf.
After a bit of tweaking in a graphics program, here's the same picture, with the veins highlighted. Notice the way they seem to spread in similar directions but actually differ from each other.