difference between octomap and voxel grid?

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hello everyone, I am new in ROS, and I would like to know the defference betwen octomap and voxel_grid ? I have looking for information on internet but I have find the same information for both of them. I have seen these videos about octomap and voxel_grid on youtube ( watch?v=y5tGI3Fln_4 and watch?v=y5tGI3Fln_4 )

If someone can explain the defference to me, or guide me to a webpage ?

thank you,

Guillaume