good insights in here
Giveaway - a donation to any warm body who may or may not ask for it (ex: Authorship, Masternodecoin, HOWL)
Airdrop - giveaway of one asset to owners of a separate asset (ex: CLAMs)
Sharedrop - airdrop in proportion to the amount of a separate asset (ex: OmiseGo, Byteball, Bitcore)
Sigdrop - signature campaign giveaway (ex: DeepOnion)
BCH is not an airdrop, it's a fork.
All new coins should do airdrops (and sigdrops too if the coin doesn't have big whales behind it), even if they are smallish (like OMG). Airdrops increase distribution, instantly creates allies, and the airdropees will start pumping your coin. Most legit airdrops are sharedrops. The easiest coins to airdrop on right now are Ethereum and Waves (bitcoin does not have a native colored coin like Ethereum does with the ERC20 token, so to get a bitcoin sharedrop you have to sign a message typically).
Most giveaways are garbage. However there are diamonds in the rough: NEM, Decred, and Ripple giveaways turned out to be huge. One problem with giveaways is transparency--how do we know that the devs didn't create a bunch of sockpuppet accounts? NEM has been accused of having many sockpuppet accounts in its giveaway.