Conglomerate is a siliciclastic sedimentary rock consisting of a mix of large & small grains (it is poorly sorted). The larger grains (pebbles or cobbles or boulders) in conglomerates are rounded to subrounded in shape. The finer-grained matrix is usually sand or mud.
Shown below is a quartzose conglomerate, having rounded quartz pebbles and quartz sand. Some conglomerates are feldspathic - they have an abundance of K-feldpar pebbles. Some conglomerates are lithic - they have an abundance of large clasts composed of a variety of other rock types.
Most conglomerates were deposited in alluvial fan environments, in many stream/river environments, and in some very shallow marine environments.
Conglomerate (quartzose conglomerate)