Subject: Can you explain weak nuclear force?

I understand that there are four fundamental forces: gravity (the weakest 
force) is between any two particles of matter, electromagnetism is the 
force between two charged particles, strong nuclear force (the strongest) 
is the force between two nuclear particles that holds them together, and 
then weak nuclear force has been described as the force responsible for 
nuclear decay (or is it just a certain kind of decay?).  But weak nuclear 
force seems to be defined differently from the other forces.  I don't 
understand, if this is a force, then what two things is this a force 
between?  And then how does that contribute to nuclear decay?
Thanks.

Re: Can you explain weak nuclear force?