Neutral & Ground bond in my barn

I am running a line (220) from my house (main control box) to my barn. I am a little confused by the "Box's" I need, meaning the box at the house is the main junction that sends out and controls the circuits to various things such as lighting, well, kitchen, etc.. The line will run from this box to a box in the barn (100 Amp, 6 circuits) that will be fused circuits such as lightning and various other lines. Is the box I am talking about at the barn called a distribution box, a main lug box or what? My reason for asking is when I view them at Lowe's they have 100 Amp boxes from $20.00 to $50.00, what is the difference? Please expound to your hearts content. Changeling

I am going to put in a exhause fan in my bathroom. All that is there now is one swith controlling 2 lights. My question is can i use the power from that swith for the fan and make it a 2 switch box. Installation I understand but it's whether I need to get power from somewhere else or not. Any input would be great.