Lagrangian and Eulerian specification of the flow field
Lagrangian and Eulerian are two ways of looking at how water moves in a stream. They both describe where the water is going, but they look at it in different ways.
The Lagrangian way looks at water as if it's a person. Imagine you're in a boat, and you're floating down a river. As you move, you can see all the places you have been and the places you have yet to visit. That's what Lagrangian does, it looks at water as if it's a person moving through the river.
The Eulerian way looks at the water from a fixed point. Imagine you're standing on the side of the river, and you can see the river flowing by. You can see the different places where the water is coming from and going to, but you don't get to know how the water got to each place. That's what Eulerian does, it looks at the river as a whole and shows where the water is entering and leaving, even if it doesn't get to look at the individual movements.
The Lagrangian way looks at water as if it's a person. Imagine you're in a boat, and you're floating down a river. As you move, you can see all the places you have been and the places you have yet to visit. That's what Lagrangian does, it looks at water as if it's a person moving through the river.
The Eulerian way looks at the water from a fixed point. Imagine you're standing on the side of the river, and you can see the river flowing by. You can see the different places where the water is coming from and going to, but you don't get to know how the water got to each place. That's what Eulerian does, it looks at the river as a whole and shows where the water is entering and leaving, even if it doesn't get to look at the individual movements.