Hey @greyman, I'm glad you were able to pick up something useful from the post. Well, yeah! Reputations can go up or down when you recieve an upvote or downvote respectively.
There are many things that can make your reputation to go down most of which involves annoying those who have higher reputation or steem power than you. Eg trolling, spamming, insulting, using the wrong tags, etc. But to be on a safer side, respect everyone within reasonable limits though. Before I forget "vote begging" can also bag you a downvote coz its a form of spamming.
How to know when you are downvoted:
Take a look at this picture. It shows the people who upvoted this particular post.
Notice the + sign infront of the names.
If you see a - sign, then the post was downvoted.
Hope I answered most if not all of your questions. Feel free to ask more if need be.
PS: there's nothing like a "poster/non-poster" on steemit. If you are here, you may want to train yourself to write about your passions. A 300 word short piece will be okay for starters.
Keep steeming!
upload png google
Thx. I never noticed the + and - before. Learned something new.