Guided Tours and Open Houses are both great ways to get to know a participating community or site. The main difference is in how each is facilitated.
A Guided Tour is a synchronous event, frequently in the format of a webinar, during which a community manager or member shows the participants the features, functionality, and activity of his or her community. The host of the Guided Tour may choose to highlight typical ways in which members engage and interact, as well as tips and tricks for effectively participating.
An Open House can be synchronous or asynchronous. A community or website is "opened" for participants to tour via a unique URL, and a chat application is available at the top of each page so that participants can chat with one another and/or community leaders and members during the tour. Open Houses can be scheduled for long periods of time for participants to view at their leisure, or scheduled for finite periods of time during which community leaders and members are available to chat with participants.