XMVector3Length returns the length of the vector.

XMVector3LengthSq returns the squared length of the vector.

If the length of the vector is e.g. 13, then the squared length is 13 * 13 = 169.

XMVector3Length is just square root of XMVector3LengthSq.

Thus:

XMVector3Length gives actual length, but isn't as fast.

XMVector3LengthSq is faster, but doesn't give actual length. It can still be used for various calculations.

As an example, these should give the same result:

1) if (XMVector3Length(myVector) < 5)

2) if (XMVector3LengthSq(myVector) < 25)

Just like Lactose! said, in general, use squared lengths whenever possible. Which tends to be just for comparing...