NTFS (New Technology File System) is a very powerful file system provided with NT versions of Windows (Windows-NT, Windows-2000, Windows-XP, Windows 7, …). Even if Windows still supports FAT, the Windows system disk is often formatted in the NTFS format.
Linux has offered full read support for NTFS partitions for a long time, however, the NTFS write support that comes with the kernel is still partial. If you want a full read-write support for NTFS disks, you will have to use ntfs-3g (Ntfs third generation driver), that is a userspace program.
If you just want a read support on NTFS disks, you just have to mount it with the following command (you have to replace xxx with the device name of your disk): mount -t ntfs /dev/xxx /mnt/windows -o ro This read-only support is safe (you cannot alter or damage your data) and may be used for example if you just want to backup several files, or read a document.
If you really need a complete NTFS Write support, you will have to use ntfs-3g. It is very easy to use: ntfs-3g /dev/sda1 /mnt/windows
In this example, we suppose the device name of the NTFS partition is /dev/sda1 and the mount point is /mnt/windows. If you do not know the device name, you should list partitions with cat /proc/partitions or use GParted.