When a post pays-out, the author gets 50% of the rewards and the 'curators' (those who upvote the post) split the remaining 50% in proportion to the stake they have in their upvotes. This 'stake' is determined based on their effective HivePower (HP) and the 'weight' at which they have upvoted the post.

For example: You write a post that earns 1.20 in rewards. You get 50% of that, or 0.60 as an "author reward." Then say that I have 1000 HP and vote your post at 100% of my Power, while Jane has 500 HP and upvotes your post at 100% of her power. Curators get 50% of the rewards on a post, but my HP is twice what Jane's is. So, my "curation reward" is 0.40 and Jane's is 0.20, for a total of the other 0.60 from the posts' rewards.

The mix of HBD -vs- HP given at payout is determined when the post is published. It can be 50% HBD + 50% HP, or it can be 100% HP, depending on the author's preference. (This can be changed in the post Editor screen under "Advanced Options.")

...and one last thing about payout...

Although rewards are usually paid in HBD/HP or HP, there is an occasional situation where HIVE is also involved in payout, based on the subtle dance of the economics of the blockchain. When one compares the marketcap of HBD with the marketcap of HIVE, the "debt ratio" is given. That ratio must not exceed 10% for the blockchain to remain healthy. When the blockchain gets close to that debt limit, it stops printing HBD and rewards are paid in HIVE and HP, instead. When the debt ratio drops again, we will begin to see HBD in rewards once more.

NOTE: Rewards are always paid in full, regardless of whether they're paid in HIVE, HivePower, or HBD!