Sedimentary rock is a property of rock pressed tightly together, and when you slice it in half, you will see beautiful layers are many different rocks pressed down together. Most mountains are made by sedimentary rocks.
Metamorphic rock is a property of rock that has been formed by pre-existing rocks. This process took more than a thousand years to form this kind of rock, and as it is made out of pre-existing rock, it is very special and old.
Igneous rocks are formed by heat with magma, also known as lava. As you can see here, this image has an igneous rock that has thousands of bubble-popped holes in them. This is because of the heat that made the "lava" bubble.