  We consider $N$-soliton solutions of the KP equation,
(-4u_t+u_{xxx}+6uu_x)_x+3u_{yy}=0 . An $N$-soliton solution is a solution
$u(x,y,t)$ which has the same set of $N$ line soliton solutions in both
asymptotics $y\to\infty$ and $y\to -\infty$. The $N$-soliton solutions include
all possible resonant interactions among those line solitons. We then classify
those $N$-soliton solutions by defining a pair of $N$-numbers $({\bf n}^+,{\bf
n}^-)$ with ${\bf n}^{\pm}=(n_1^{\pm},...,n_N^{\pm}),
n_j^{\pm}\in\{1,...,2N\}$, which labels $N$ line solitons in the solution. The
classification is related to the Schubert decomposition of the Grassmann
manifolds Gr$(N,2N)$, where the solution of the KP equation is defined as a
torus orbit. Then the interaction pattern of $N$-soliton solution can be
described by the pair of Young diagrams associated with $({\bf n}^+,{\bf
n}^-)$. We also show that $N$-soliton solutions of the KdV equation obtained by
the constraint $\partial u/\partial y=0$ cannot have resonant interaction.
