serial voting was reversed
This means that a user visited a bunch of your questions/answers in a row, and the system flagged it as a serial voting. This is to prevent voting spam, such as one user discovering another user's questions/answers and deciding to upvote/downvote many values at once. This might be a collaboration (lots of upvotes) or revenge (lots of downvotes). Generally, there was no change in your reputation; it was simply added then removed (i.e. there is a corresponding +25 that counters the -25).
user was removed
This message means that a user was deleted, either at their request or because it was a spammer, so all of their votes have been removed. Again, this is perfectly normal, and simply means the 250 points that user contributed to you was reversed (i.e. there was a +250 elsewhere in your history from this user).
It's a bit disconcerting at first (I've had it happen a few times), but it's not a reflection on your contributions to the community, but simply the system working as intended to keep reputation fair. As you continue to interact with the community, your reputation will naturally rise as you ask good questions, give good answers, and accept answers.