Joining Guilds - Open/By invitation

What is the difference between the two ways of joining a guild? I am specifically trying to find out whether, if you have your guild set to 'Open', this means anyone can just join or whether they can only ask to join (as opposed to being invited to join). In other words, in an Open guild scenario, if you click join, does the Leader get a request that he then has to accept? Or do you simply walk right in?
If your fans meet the criteria you just walk in if its an open guild.

If its invite only, the leader, his right hands and all experienced members can send invites