If you do not cast a vote on a certain race or measure it is simply considered an undervote. An undervote does not invalidate the rest of your ballot. Those races and measures you do vote on will still count.
Overvotes
If you vote for more candidates than allowed, or if you vote both Yes and No on a measure, it is called an overvote. Your vote will not count for that candidate or measure.
Make sure that you read the instructions on your ballot.