Waves are large bodies of water that form a curved shape and carries energy. Waves are just one type of carrying energy. There are many more type of carrying energy
Longitudinal waves are sound waves, they are motion of a medium is parallel to motion of a wave. There are sound waves all around you, they are around when people or animals or other thing talk or make noise.
Electromagnetic waves are the particles in light. When you shine a flashlight, or turn on a lamp the light from the flashlight and lamp has particles and waves that are called electromagnetic waves
Wave Crests are the highest point of a wave. There is the lowest the start the length and waves crests are the highest point.
A medium is any substance that a wave travel through. These dots are mediums.
Wave trough are the lowest point of a wave. There is the height the start the length but wave trough is the lowest point