A black hole in simple terms is a point in space where the gravity is so dense that light cannot escape its grasp.
Singularity
The singularity is the center of the black hole where the gravity is at its densist.
Event Horizon
The event horizon is the surface of the black hole. What happens when this is crossed is quite interesting. The singularity inside of a black hole has significantly much more gravity than the event horizon. Therefore, when the event horizon is crossed anything inside the black hole will be susceptible to greater gravity that what is outsie the black hole. This phenomenom is known as spaghettification since you would be strecthed thinner than a piece of spaghetti.