They are held as a full interest for a specified period of time. For the times you use it, the interest is undivided.

As an owner of an undivided interest in real property you have the right to the use and possession of the whole property. If there are three owners and the property is sold or partitioned you have the right to one-third of the net proceeds. You have the right to one-third of the profits.

Typically yes

Yes. You are free to sell you interest to any willing buyer.

No. An undivided interest in property means that two or more persons own the real estate and each has the right to the use and possession of the entire property even if they own only a one-half undivided interest.