neek is a NAND emulation environment where the console is tricked to run with a fake "internal wii memory", which is in fact located on an external device like in Sd (Sneek) or Usb (Uneek).
as it can be "S" for Sneek, or "U" for usb, users often use "x" to tell "whatever version" xNeek.
in neek environment, everything is running from the emulated (redirected) NAND, no access is done on your Real internal NAND memory.
It's useful to test things like channel creation/edition, if you brick your Wii ... it's in fact the copy of your Wii that you broke, your Wii is still fine.
It's often use to play WiiWare and VC, as you can install a lot more games, you are not limited by the 528MB space, you can store up to 2TB of data.