fdisk is the system command used in FreeBSD and other unixlike OSes to handle partitioning hard drives. (See also disklabel for editing disk slices.) Along with disklabel, this fdisk provides roughly analogous functionality to the same-named Windows command fdisk.
It is worth noting that fdisk, and for that matter FreeBSD in general, does not allow editing of the partition tables on any drives that have a partition or slice mounted. One should boot from a rescue disk when editing the partition table on the hard drive containing your root (/) mount point.
See also sysinstall for accomplishing all hard drive partitioning, slicing, and formatting tasks inside a convenient text-mode GUI.