We consider the semi-linear elliptic PDE driven by the fractional Laplacian: \begin{equation*} \left\{% \begin{array}{ll} (-\Delta)^s u=f(x,u) & \hbox{in $\Omega$,} \ u=0 & \hbox{in $\mathbb{R}^n\backslash\Omega$.} \ \end{array}% \right. \end{equation*} An $L^{\infty}$ regularity result is given, using De Giorgi-Stampacchia iteration method. By the Mountain Pass Theorem and some other nonlinear analysis methods, the existence and multiplicity of non-trivial solutions for the above equation are established. The validity of the Palais-Smale condition without Ambrosetti-Rabinowitz condition for non-local elliptic equations is proved. Two non-trivial solutions are given under some weak hypotheses. Non-local elliptic equations with concave-convex nonlinearities are also studied, and existence of at least six solutions are obtained. Moreover, a global result of Ambrosetti-Brezis-Cerami type is given, which shows that the effect of the parameter $\lambda$ in the nonlinear term changes considerably the nonexistence, existence and multiplicity of solutions.