Compaction is the process in which peices of sediment are squished together (or compacted) by the weight above them. This happens with sedimentry rock because it has layers. The weight of the top layers pushes on the lower layers squeezing everything together, and making the layers solid.
Cementation
Cementation is the process in which peices of sediment are heald together by another material. Dissolved minerals create a natural substance that acts like cement, sticking the larger peices of sediment together.