#3
if you place a capo, lets say, on the 2nd fret, you play the same music as if the capo was the nut. and don't double post...
#4
capos are used to make the guitar pitch sound higher
it is put on any of the frets you want and takes the place of the nut
its sorta like moving the nut up higher on the guitar

and yeah dont double post
#5
One of the major uses of a capo is to raise the pitch of all the strings simultaneously to match that of the singer. People have different vocal ranges, so not everyone can sing in the same key that a standard guitar is tuned to. So a capo is used to raise the key to match that of the singer and song. Then you can still play as normal, it's just that much higher sounding is all. Trying to uptune to those notes would certainly snap strings, so the capo was invented. One of the most famous songs to use one is Hotel California by the Eagles. Place it on the 7th fret for the main melody of the song, and it's especially sweet when done on a 12 stringer. For us lowly humans, we need to use the capo for that song, however I've seen the live stuff by them and they are just barring across all the strings. Makes sense tho since they're way up there in that God-like status of musicians, and well, it is THEIR song after all. lol