Matrix notation question || and x

I want to calculate the radius of a circle given three points on its perimeter. So I find this formula:

Circle - Wikipedia, the free encyclopedia

|P1-P2| |P2-P3| |P3-P1|

------------------------- = r

2 |(P1-P2)x(P2-P3)|

where the P's are 1 by 3 vectors with x,y,z coordinates of the points 1,2,3.

What does the "|" mean in the matrix world? Isn't that the length of the vector in geometry?

And the "x" oepration? Is that matrix multiplication? Then how could that be performed on two 1 by 3 vectors???

Thankful for clarification!