How can I move all the files from one folder to another using the command line?

You can use move for this. The documentation from help move states:

Moves files and renames files and directories.

To move one or more files:
MOVE [/Y | /-Y] [drive:][path]filename1[,...] destination

To rename a directory:
MOVE [/Y | /-Y] [drive:][path]dirname1 dirname2

  [drive:][path]filename1 Specifies the location and name of the file
                          or files you want to move.
  destination             Specifies the new location of the file. Destination
                          can consist of a drive letter and colon, a
                          directory name, or a combination. If you are moving
                          only one file, you can also include a filename if
                          you want to rename the file when you move it.
  [drive:][path]dirname1  Specifies the directory you want to rename.
  dirname2                Specifies the new name of the directory.

  /Y                      Suppresses prompting to confirm you want to
                          overwrite an existing destination file.
  /-Y                     Causes prompting to confirm you want to overwrite
                          an existing destination file.

The switch /Y may be present in the COPYCMD environment variable.
This may be overridden with /-Y on the command line.  Default is
to prompt on overwrites unless MOVE command is being executed from
within a batch script.

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See the following transcript for an example where it initially shows the qq1 and qq2 directories as having three and no files respectively. Then, we do the move and we find that the three files have been moved from qq1 to qq2 as expected.

C:\Documents and Settings\Pax\My Documents>dir qq1
 Volume in drive C is Primary
 Volume Serial Number is 04F7-0E7B

 Directory of C:\Documents and Settings\Pax\My Documents\qq1

20/01/2011  11:36 AM    <DIR>          .
20/01/2011  11:36 AM    <DIR>          ..
20/01/2011  11:36 AM                13 xx1
20/01/2011  11:36 AM                13 xx2
20/01/2011  11:36 AM                13 xx3
               3 File(s)             39 bytes
               2 Dir(s)  20,092,547,072 bytes free

C:\Documents and Settings\Pax\My Documents>dir qq2
 Volume in drive C is Primary
 Volume Serial Number is 04F7-0E7B

 Directory of C:\Documents and Settings\Pax\My Documents\qq2

20/01/2011  11:36 AM    <DIR>          .
20/01/2011  11:36 AM    <DIR>          ..
               0 File(s)              0 bytes
               2 Dir(s)  20,092,547,072 bytes free

 

C:\Documents and Settings\Pax\My Documents>move qq1\* qq2
C:\Documents and Settings\Pax\My Documents\qq1\xx1
C:\Documents and Settings\Pax\My Documents\qq1\xx2
C:\Documents and Settings\Pax\My Documents\qq1\xx3

 

C:\Documents and Settings\Pax\My Documents>dir qq1
 Volume in drive C is Primary
 Volume Serial Number is 04F7-0E7B

 Directory of C:\Documents and Settings\Pax\My Documents\qq1

20/01/2011  11:37 AM    <DIR>          .
20/01/2011  11:37 AM    <DIR>          ..
               0 File(s)              0 bytes
               2 Dir(s)  20,092,547,072 bytes free

C:\Documents and Settings\Pax\My Documents>dir qq2
 Volume in drive C is Primary
 Volume Serial Number is 04F7-0E7B

 Directory of C:\Documents and Settings\Pax\My Documents\qq2

20/01/2011  11:37 AM    <DIR>          .
20/01/2011  11:37 AM    <DIR>          ..
20/01/2011  11:36 AM                13 xx1
20/01/2011  11:36 AM                13 xx2
20/01/2011  11:36 AM                13 xx3
               3 File(s)             39 bytes
               2 Dir(s)  20,092,547,072 bytes free

move c:\sourcefolder c:\targetfolder

will work, but you will end up with a structure like this:

c:\targetfolder\sourcefolder\[all the subfolders & files]

If you want to move just the contents of one folder to another, then this should do it:

SET src_folder=c:\srcfold
SET tar_folder=c:\tarfold

for /f %%a IN ('dir "%src_folder%" /b') do move "%src_folder%\%%a" "%tar_folder%\"

pause