What are all the event types and what do they mean?
upvote / downvote - Your post was upvoted or downvoted.
unupvote / undownvote - An upvote or downvote previously cast on the post was retracted.
downvoted - Note the extra "d" at the end; you cast a downvote on an answer. Downvotes you cast on questions are not shown in your reputation history and do not affect your reputation.
accept / unaccept - Your answer was accepted or unaccepted by the OP, or you accepted an answer on one of your questions.
bounty - You offered a bounty on a question or a bounty was awarded on your answer.
removed - Most often, the post was deleted (this could be any post you took action on, not necessarily a post you owned). Keep in mind that when a question is deleted, all of its answers are automatically deleted as well. This event is also used when a user is removed. Note that when a post gets deleted, the question's title will be shown in plain text to those who do not have access to view it.
migrated - Your post was migrated to another site.
edit - An edit you suggested to a post or tag wiki was approved.
reversal - Another user serially voted on your posts, which got reversed.
Note: Currently the downvote and downvoted events are switched on the mobile version.
Why is there an unexpected or no reputation change next to the event?
Most often it's due to you hitting the reputation cap. You can only earn up to 200 reputation from votes and edits in a single day. Sometimes the last event that puts you up to the limit will be slightly less than what you'd normally find, such as a +6 for an upvote to your answer. After hitting the cap, all additional events will show no reputation change.
Additionally, any upvotes you get to community wiki questions or answers you have will still show up in the reputation history, but will not show any reputation change next to them since they are not eligible for reputation gain.
Events in your reputation history are also wrapped up into a single line when they occur on the same day. If you are viewing your history by time, this can cause some confusion as it might appear that you have received additional reputation for a new upvote that just occurred, when in fact you didn't. See:
This is because that single post will be listed by the time of the last event on your post with the sum of all events on that post, even if you didn't gain any additional reputation for that event. If you find this confusing, try switching your reputation history view to the post option. This will allow you to expand each post to see all the separate events for it, including how many gained you reputation and how many didn't.
Why are events sometimes highlighted in yellow?
This represents new events that you haven't seen yet regarding your reputation. They are only highlighted for that single view of the page. If you leave and return to your reputation history, they will no longer be highlighted. The total amount of the highlighted events should add up to the number in the bubble of the reputation tab itself (excluding any removed and reversal events). This bubble is meant to indicate the total amount your reputation has changed since you last checked the reputation tab. It will not appear until you've visited the tab at least once.
My reputation dropped, but I don't see anything listed!
This can be caused by one of several different reasons:
Removed posts are hidden. More than likely, you don't have your reputation history set to show removed posts. By default, removed posts are hidden from the history pages. In order to show removed posts, you need to make sure the checkbox all the way at the bottom is checked.
Reversal events from the same day. When you receive an upvote and a subsequent unupvote event (or an accept and an unaccept, or a downvote and an undownvote) from the same user on the same day, the events collapse into each other and are not displayed in your reputation history. This was primarily done to prevent users from spamming a user's reputation history with consistent events reversing each other. A reversed event (unupvote, undownvote, unaccept) will only appear if it occurs on a day other than the day the original event occurred.
Background processes. If you're still unable to see what caused a sudden reputation drop, there may have been a system process in the background. This usually happens when two or more accounts are merged together.
Why would an event disappear from my reputation history?
This should only occur for upvotes, downvotes, and accepts. Whenever one of these actions gets reversed during the same day, the event is just completely removed from your history rather than creating a separate unupvote, undownvote, or unaccept event in the list. This was done to prevent users clicking a vote on/off repeatedly to fill up a user's history with useless events.