  We study the homogeneous Cauchy-Dirichlet Problem (CDP) for a nonlinear and
nonlocal diffusion equation of singular type of the form $\partial_t u
=-\mathcal{L} u^m$ posed on a bounded Euclidean domain
$\Omega\subset\mathbb{R}^N$ with smooth boundary and $N\ge 1$. The linear
diffusion operator $\mathcal{L}$ is a sub-Markovian operator, allowed to be of
nonlocal type, while the nonlinearity is of singular type, namely
$u^m=|u|^{m-1}u$ with $0<m<1$. The prototype equation is the Fractional Fast
Diffusion Equation (FFDE), when $\mathcal{L}$ is one of the three possible
Dirichlet Fractional Laplacians on $\Omega$.
  Our main results shall provide a complete basic theory for solutions to
(CDP): existence and uniqueness in the biggest class of data known so far, both
for nonnegative and signed solutions; sharp smoothing estimates: besides the
classical $L^p-L^\infty$ smoothing effects, we provide new weighted estimates,
which represent a novelty also in well studied local case, i.e. for solutions
to the FDE $u_t=\Delta u^m$. We compare two strategies to prove smoothing
effects: Moser iteration VS Green function method.
  Due to the singular nonlinearity and to presence of nonlocal diffusion
operators, the question of how solutions satisfy the lateral boundary
conditions is delicate. We answer with quantitative upper boundary estimates
that show how boundary data are taken.
  Once solutions exists and are bounded we show that they extinguish in finite
time and we provide upper and lower estimates for the extinction time, together
with explicit sharp extinction rates in different norms.
  The methods of this paper are constructive, in the sense that all the
relevant constants involved in the estimates are computable.
