1) Are formed from layers of sediment (rock fragmnets or dead matter) laid down in lakes or seas over millions of years.
2) Sedimantary rocks can also form when water evaporates and leaves a dissolved solid (like salt) behind. The layers are cemented together by other minerals.
3) Fossils can form in sediments. These are the long dead remains of plants and animals. The type of fossil is used to work out the relative age of the rock.