How the Act of Upvoting Directly Affects My Wallet and My Ability to Upvote

On https:// steemit.com/faq.html ( https://steemit.com/faq.html ) I found:

[quote]
Voting and curating - If you discover a post and upvote it before it becomes popular, you can earn a curation reward. The reward amount will depend on the amount of STEEM Power you have.
[end quote]

(1) How is "popular" defined?
(2) In what units are curation rewards denominated?
(3) Is the number of posts or comments that I upvote limited, and if so, limited by what and for how long?

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