Reverb is characterized as random, blended repetitions of a sound occurring within thirty milliseconds after the sound is made. This is all the sound that immediately bounces off any nearby surfaces before it gets back to your ears. So first you'll hear the original sound and then all the stuff bouncing off the walls, furniture, trees, people etc. Your brain is specially equipped to notice reverberations before your conscious perception does. It blends the reverb with the original sound before you even notice it and then tells you that it all came from the same location, just so you don't get confused.
Snap your fingers, make a sharp clicking sound with your mouth, and try to listen to the room around you instead of yourself. You'll start to hear it. Try it in different rooms. It'll sound different each time.
Echo is defined by distinct repetitions of a sound occurring after 30 milliseconds. This is when you can unquestionably hear a distinct... well, echo of a sound coming back to you. When you are at a big canyon or inside a gigantic room and you, of course, say "Hello!" and then a moment later you hear it again, that's echo.
Hope this helps.