A “blockchain hard fork” is an event where there is a split in the network with two competing blockchains. This may take the form of:
Business as usual, where in the normal course of operation, temporary network splits happen and resolve themselves (typically within seconds or minutes); no new currency or blockchain is created.
Non-contentious upgrades hard forks, where typically the majority of a network upgrade to a new version of the software and the old chain dies off; no new currency or blockchain is created.
Non-contentious “airdrop” forks, where a new blockchain is created, splitting off from an existing blockchain. Both chains continue to operate normally under different names (for example Bitcoin and Bitcoin Cash). In this case, an additional blockchain is created, and operates under a new name.
Contentious fork, where segments within the community cannot reach consensus on the rules of the blockchain. One or both sub-communities implement changes to the blockchain and accepting that “their” version of the consensus rules represent the “new” chain. In this case, there may be significant disruption to the network, and one or multiple competing currencies may remain in operation after the event. There may even be industry confusion as to the naming of each blockchain.
An “Air Drop” is where tokens are allocated to existing token holders in a blockchain. The newly created tokens may be allocated in a fork of the source blockchain, or they may be tokens in a new or different blockchain.
Air Drops are typically non-contentious; normally some steps must be undertaken for a wallet management service to obtain the airdropped tokens.