Difference between state estimate data and odometry data

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I am trying my hand at robot navigation. I am not able to comprehend, the following differences,

1. Difference in the odometry data from sources like wheel or visual and the output generated by state estimators like robot_pose_ekf packages. Because they both seem to have the same message format.

2. I am able to understand the transform between baselink and different sensor coordinate frames. But what is the difference between world, map, nav, odom and base_link frame. In some cases I find, base_footprint and base_stabilized coordinate frames too.

3. In the tf map, should there be one fixed frame and can odom_frame be made as fixed frame in the presence of /map or /world frame?