Remember how much you loooonged to have roommates in all those hard-earned homes, cottages, and tree houses? You had friends you wanted to be able to drop by whenever they felt like it (whether they were online or not), loved ones to share all of your possessions with, and appointed caretakers to feed and pamper your piggies while you were away.
Well now, we've made all that possible. To share your home with others, all you need is to give 'em a key. It's pretty darn simple. You now have a keyring icon that lives to the left of your inventory. This is where alll of the magic happens: the keyring menu that will let you share a key with a friend (if you have a house) and view keys others have shared with you.
Anyone with a key to your house can (1) enter it at any time, and (2) have access to everything in your house (cabinets, gardens, etc), with the exception of icons.
To make the pilgrimage to a friends home you have a key to, you'll have the option of either setting it as a destination or teleporting right over.
So try it out! Put down the proverbial welcome mat and let someone in.