HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

Changes that affect gameplay?

Yes, but mostly not significantly. For example, recently someone mentioned that the levels at which Conflux mages gain wisdom and elements have been changed: in the original, it was 6-4, like Warriors, but in HD+, it's 3-3, like all other mages. There aren't a huge number of such changes, but there are a noticeable amount.
There are also larger changes, for example, fixing the quick-cast bug with the Hydra in the Hive.