Voting power is based upon how much Steem Power (aka vesting STEEM) an account holds as well as how much voting an account has done recently. The Steem network attempts to approximate your average voting rate such that your voting power on any given post is approximately:

SteemPower / VotesPerDay

Because Steem doesn't know how many votes you will cast in the near future, the VotesPerDay calculation is a moving average. This means each consecutive vote you make will increase your average VotesPerDay. If you go 24 hrs without voting then VotesPerDay will get reset to its minimum value: 20.

You can increase your voting power by posting and voting. 50% of all content and curation rewards are paid out as SteemPower.

The amount of benefit a vote confers on the authors of a post depends upon how many votes are already cast. This is because payout is determined by the following calculation:

NewPayout = (OldVotes + NewVotes)^2
This means that small votes have a much greater absolute benefit when added to a post with a lot of votes, and almost no apparent benefit when added to a post with no votes.

InitialPayout 1 = (0+1)^2
AfterSecondVote 4 = (1+1)^2
The outside observer would think that the second voter counted for "3" even though both voters were of the same weight. The reality is that if a 3rd down vote was cast by an another voter of equal weight the Payout would fall to 1. A fourth voter downvoting would return the payout to 0