Here's a breakdown:

* Sedimentary rocks are formed from the accumulation and cementation of sediments, which are fragments of other rocks, minerals, or organic matter.

* Gravel is a type of sediment made up of large, rounded fragments.

* Sandstone is a sedimentary rock composed primarily of sand-sized grains. Some sandstones can contain gravel-sized particles.

* Conglomerate is a sedimentary rock composed of rounded gravel-sized fragments cemented together.

So, while they differ in grain size and composition, all three are classified as sedimentary rocks because they are formed from the accumulation and cementation of sediments.