first you should know what fragmentation is:
I'll try to illustrate this with an example...
imagine that you have a section of your hard disk, we'll illustrate this as being empty with twenty dashed lines
--------------------
now imagine that you write a file that's five units, (bytes, MB whatever you want to think of it as, (file A)
the disk now looks like this
AAAAA---------------
now you write file B, that is 2 units big
AAAAABB-------------
now you delete file A
-----BB---------------
now you write file D (2 units)
DD---BB---------------
now write file E nine bytes
DDEEEBBEEEEEE-------
now delete file B
DDEEE--EEEEEEE------
Write file F (four units)
DDEEEFFEEEEEEFF----
you can see how bits of files are all over the place, because the disk will fill the disk in a sensible way, even if this means splitting up the files (fragmenting them) over differetn areas of the disk...
so back to your questions,
1, a defrag, is de fragmenting the files, in the above example the disk looked like this.
DDEEEFFEEEEEEFF----
a defragment will go through several steps moving files in a logical order.
DDEEEFFEEEEEEFF----
put file F all together
DDEEE--EEEEEE--FFFF
move file D
--EEE--EEEEEEDDFFFF
bunch file E together
EEEEEEEEE----DDFFFF
then perhaps put it all at the start of the disk leaving all the free space at the end
EEEEEEEEEDDFFFF----
2, there is no such thing as too often, but you don't need to do it all the time as the disk may not be fragmented enough to make a difference
3, when files are fragmented the disk head needs to scan across the disk to be able to find all the parts of a file, and when it writes it needs to scan all across the disk to find the free space. you if you reduce the amount of time taken to read/write files, yes your computer will appear to be marginally faster/smoother
4, no, windows won't do it automatically, why would it? I certainly wouldn't want to find that it was taking even longer to read and write files because windows had decided for itself that it'd like to do a little house keeping.
5, you can schedule it if you like, or you can decide to run it as and when you want to do it, personally I don't see the need to schedule it so I've never looked at how to do this. it's the kind of thing that you'll only really run once a year or so, or more often if you;re finding that things are going really slowly doing file operations
6, there is one built into windows, (right click the disk in my computer select properties, go to the tools tab). some people prefer to use third party tools and say that they work better, I can't comment as I've never tried,
what I would say is that sometimes running a defrag once doesn't de fragment files as much as you;d like so you may wish to run the de fragment process multiple times till all the disk turns blue and there are no red lines left indicating fragmented files.
7, no there are no dangers, you're not likely to loose data, of course if the power fails during a write operation then you may loose data, or corrupt the disk, but this could happen at any time.
