Explain Basic Supply Demand Graph Can someone please explain these things?  They show up all over the place and it seems people can't be bothered with documenting the labels.  Is this showing f(L) = W?  Is it showing f(W) = L?  If so, what's up with the labels (D) and (S) on the curve?  Is something else going on?  What's the dependent variable?  Are we trying to describe f(x,y) = S with a 2D graph?

Confused.
Assuming that $W$ is wage and $L$ is labor, I would say this is a labor supply-demand graph. TD stand for demand and S for supply. 

The graph shows a positive shift in labor supply, i.e. labor offered by workers, leading to an increase in $L$ and a decrease of $W$ due to the excess of supply. 

The shift in labor supply can be caused by a variety of shocks as a preference shock which values more labor, a change in non-labor income, etc (look here for explanations)