What does NSF certification cover on this filter?

The listing states it is NSF certified as a kitchen water purifier. Certification applies to the claims named on the certification, so check the specific standard listed for this model before relying on it for a particular contaminant.

Does it fit a standard kitchen faucet?

It is a faucet-mounted filter designed for typical kitchen faucets. Faucets with pull-down sprayers or non-standard threads may need an adapter.

What filter does it take?

It uses the FF-06-3 filter, and one extra is included in the box. Replace it on the schedule Waterdrop gives for the FF-06-3.

How is it different from an under-sink reverse osmosis system?

This is a single-stage carbon faucet filter. Reverse osmosis removes dissolved contaminants such as many minerals and metals that carbon alone does not, but RO needs under-sink installation and space.

Does the 360° rotation affect water flow?

The rotating head lets you swing the filter body aside. Faucet filters generally reduce flow compared with an unfiltered tap, so expect a slower stream.