Multiple Square Bracketed Function definition

Question

I am familiar with the typical way of defining a function--such as

f[x_,y_]:=x+y

but I have not found a discussion of definitions such as

torus[a_,b_,c_][p_,q_][u_,v_]:= {(a+b Cos[u])Cos[v],(a+b Cos[u])Sin[v},c Sin[u]}

I am not asking about this specific example but the f[][][]:= structure as opposed to the usual f[]:= simpler one. Can someone point me to a discussion?