Sedimentary rock is eroded bits of different rocks that travel to different places in the world. After a while, it will fall in to a water source and decompiled living organisms (no longer alive) while compact and pile up. After years of deepening and compression, sedimentary rock will form and be pushed up to the surface.
Sediment is the different forms of rock that sedimentary rock is made of. It is also decompiled to a small state then, compacted to form sedimentary rock.