Sedimentary Rocks are formed on or near the Earth's surface. Sedimentary rocks belong to one of the following groups: clastic, chemical, or organic.
Most sedimentary rocks form from the weathering of igneous, metamorphic or other sedimentary rocks. Rivers erode rocks and re-deposit these sediments as new layers. These sediments can also come from the wave action of oceans or lakes, volcanic rocks and volcanic ash also form sedimentary rocks.
Another source of these sediments is from glaciers grinding away at the rocks as they move and make contact with the walls of valleys. All of these actions are the sources of clastic materials.
Chemical deposition can be by precipitation. Cave formations are a form of precipitation of a saturated as the solution looses some of its water causing the minerals to precipitate. Oolitic sands are also formed when solutions become saturated and deposit their chemicals similar to the way a pearl is formed, layer upon layer builds up on a nucleus forming little balls.
The third group of sedimentary rocks are those that are formed from organic materials. Most of the time this is the formation of peat and the series of changes that take place over time when to the peat until it becomes coal.
Fossils are found in sedimentary rocks, an animal dies and their bones sink to the bottom of a lake or are covered by sand where they may become a fossil. You can also find parts of plants, bird feathers and insects and many other materials that fossilize.
Sometime you can find some very unusual material that form fossils, one is animal poop; scientifically they are called coprolites.
Many fossils are so small they require that you look at the rock with a microscope; they are the remains of microscopic plants and animals.
Fossils can only be found in Sedimentary rocks.