What are the differences among [on hold], [closed] and [deleted] questions? I searched the meta, but I could not find the answer to this question. My apology is in order if the answer is well-known.
When $5$ users (or a moderator) vote to close a question for various reasons (e.g. duplicate, off-topic, too broad and so on), it is marked as [on-hold].
Once a question has been [on-hold] for $5$ days, it is marked as [closed].
In either case, new answers cannot be added to the question, but it is still publicly visible to all users. Users with the appropriate privileges can vote or comment on these questions.
When a post is deleted, it is only visible to users past a certain reputation threshold, and it cannot be answered, commented on, or voted on. A question can be deleted through various means: Automatically (e.g. negatively scored questions without upvoted answers can be deleted by the Community user), or via the deletion votes of 10k+ users / moderators.