The term "Whale" was originally used to describe a wealthy Bitcoin trader who has an influence on the market. It then became a widely used term for any wealthy cryptocurrency trader. 
On Steemit a whale is a user who has a lot of SP (STEEM Power) and therefore more voting power.
The term 'Dolphin' is used to describe a cryptocurrency user who does not have as much revenue as a whale but has more revenue than a minnow. This means a dolphin has some influence unlike a minnow. A dolphin is less influenced by a whales activity.
As well as describing a small fish, the term "Minnow" also means a small or insignificant person or organization.
In cryptocurrency a Minnow is a is a trader with very little revenue. A minnow has very little or no influence on the market. minnows often follow the herd so are dictated by the whales influence.
On Steemit a minnow is a user who does not own much Steem power. As such, the minnow has very little voting power and hardly any influence on the revenue distributed to the poster.
Continuing the sea ecosystem analogy some users also are denominated Orca or Redfish. Other steemians define in different ways like Steemizen, Steemulan, etc.