The earth has a core of solid iron and other heavy materials, surrounded by a liquid iron outer core. The core rotates inside the planet and creates a strong magnetic field. Outside the core is the mantle, a thick layer of molten rock called magma. Floating on the mantle is the crust, a thin layer of rocks.
The earth's crust is broken into "tectonic plates" which move around. When plates collide, they crumple up and form mountains, like the Himalayas. When the plates spread apart, magma oozes up from the mantle and hardens into new rocks. Earthquakes are caused when the edges of plates grind together, and volcanos are often found where two plates meet.
Because the plates are constantly moving around, melting, and reforming, earth's surface is said to be "geologically active". That's why the earth doesn't have very many craters compared to the moon and most other objects in the solar system. Earth does have some craters though. One of the most famous is the Barringer meteor crater in Arizona. This crater is almost a mile across and was made by a nickel-iron meteor about 150 feet across. The impact released the same amount of energy as exploding 20,000,000 tons of dynamite!
From the surface, the earth seems like a pretty big place, but in reality, it's a tiny speck. Here is a picture of the earth that was taken by the Voyager 1 probe in 1990 from a distance of 3.7 billion (3,700,000,000) miles. The earth is the tiny dot. The streaks across the image are scattered light from pointing the camera so close to the sun. Click the picture to read a quote about it.