SLR stand for Single Reflex Camera
Image for reference
1) Front Mount Lens
2) Reflex mirror at 45 degree angle
3) Focal plane shutter
4) Film or Sensor
5) Focusing screen
6) Condenser lens
From the image above, it shows the light coming in from the front lens and reflected by the mirror to the eye piece though Pentamirror. When you press the shutter button of the camera, the mirror will flip up and the light will go directly to the image sensor / film.
Well, I shouldn't be too technical about this. Basically, the SLR camera allows you to take photos like what you see from the eyepiece which is what you get on the film.
Major Difference between Point and Shoot and DSLR (My point of view)
Point and Shoot cameras are usually smaller and lighter, which allows you carry the it around. However, the larger size SLR also has it advantages. Think about when someone ask you to take photos of him/her and you have a SLR on hand, they will think, "This guy should be able to take good picture of me" or "This guy is a professional". :)
I had experience at a wedding event with my 5D Mark II on hand and others had point and shoot cameras with them. Since they think I am the professional, I always get the best position which allows me to get better photos for sure. :)