There are two methods of earning Steem on Steemit.
The first is by writing a blog post. As that blog post accrues votes, the amount of Steem that will be distributed to the writer increase.
However, every vote on the site is not worth a flat amount of money. Instead, the amount earned is based on both the number of votes an individual receives and the amount of Steem Power a voter has.
For example, if one user had 1,000 Steem Power and another had 10,000 Steem Power, it's clear that the latter has a more powerful account than the former. The effect of each of those two people voting on a piece of content is not equal; specifically, the user with 10,000 Steem Power's vote is worth more.
This has resulted in the development of a sort of whale chasing culture, whereby writers hope to convince the large Steem Power holders to give them upvotes. A vote by Scott or Larimer has been seen to increase the value of a post by hundreds of dollars, resulting in a pile-on effect where others chase the post.
To incentivize this sort of voting, there is also the curation reward paid out in the form of Steem Power. If a post does well, you earn more Steem Power than if a post doesn’t do well, thus incentivizing you to only vote for content that you believe is high quality.
Further, not every vote from the same account is equal in value. Voting on multiple pieces of content reduces the strength of each of your votes depending on how much time passes between each vote. Scott explained voting power by comparing it to certain video games.