Someone was looking to see what various focal lengths actually look like and I thought it was a great question because, while seasoned photographers have the numbers ingrained in their brains, we shouldn't forget that they are abstract to people new to photography. So what does 10mm, 20mm, 30mm and even 500mm really look like? Just look at the examples below!
Keep in mind that the distance from the camera to the tree is 10 meters (about 30 feet) and the knot on the tree that you can see towards the end of the series is 6cm (2.5 inches) across.
Also, these example photos were taken with a cropped sensor camera. Most of the mid to entry-level DSLR's have cropped sensors in them. Full frame cameras are usually found in the pro line of whatever brand you use and using one of those would produce slightly "wider" or "less telephoto" images than the examples below. But assuming you have the type of camera that most people have the examples below should be very close to what you'd see.