What are main differences between rafts, skirts and brims?
Rafts:

Rafts are a few layers of plastic a placed on the printing surface before the object is printed.  If non water dissoluble filament is used, a bit of an air gap will be placed between the raft and the print itself so it can be removed easily. A raft can help a print stick to the printing surface as it normally has a larger surface area than the bottom of the print.  It also allows the bottom layers of the print to contact another layer of plastic so there is less spreading compared to printing on the printing bed itself.



Skirts:

Skirts are loops of plastic that are extruded around the object that is being printed.  On the first layer this allows the filament to get pushed through the extruder so that a steady flow can be established.  A multiple layer skirt can be used to create a blockage around the print so there is less air movement on the print and the print can cool slower.



Brims:

A brim is a layer of plastic that is placed on the first layer of the print go allow for better adhesion to the printing surface.  It does not extend under the print as a raft does, but only goes from the edge of the print to a set distance away.  



Credits: Images from Slic3r