guesthouse = a small building, separate form a main house for the housing of guests
inn = a commercial establishment that provides lodging (I wouldn't use it here)
B&B = bed + breakfast = usually in a family's house (provides bedand breakfast for the guests/travellers, etc.

According to Aurélio, pousadas and estalagens are the same thing. Here in Brazil, pousada is a family house where you can rent a room. In Northeastern USA, this same thing is called a hostel. Estalagens may refer to lodges (like student lodges) but I am not sure.

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Note added at 13 mins (2005-10-02 17:24:04 GMT)
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Inns are quite big. The difference between an inn and a hotel is that the rooms in an inn are always close to the parking lot.

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Note added at 20 mins (2005-10-02 17:30:17 GMT)
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Inn: A group of attached or detached buildings on the same lot containing less than twenty-five (25) individual sleeping quarters for rental to the public. An operation with twenty-five (25) or more individual sleeping quarters is defined as a “motel” or “hotel.”
www.mrsc.org/mc/issaquah/Issaqa18/issluc1802.html

Motel: A type of hotel in which parking is provided at or near the room and the room door gives out onto the parking lot.
www.hometravelagency.com/dictionary/ltrm.html

In Brazil, motels have a completely different purpose.