Help wanted. How does gravity work?

Guest here, can someone explain to me how gravity works. I'm trying to code a quick platformer game in C++, but my program isn't working the way I wanted it to. My programming is flawless (and by that I mean it does what I want it to do), the only problem is that I'm having it do something that bastardizes physics. As you can probably guess, I've never taken a physics class before.

Gravity is a force that moves objects down at a certain rate, but how does the ground cancel out that force? Does the ground also create a force that cancels out gravity's force? 

Also, how would you go about creating a gravity program?

If you have a sprite affected by gravity, it will move down until the frame it moves through the ground tile.  You need to detect this, calculate the displacement required to correct the sprite to ground level, and then apply that to the sprite, then draw the screen.

Typically the ground wouldn't feel anything.  It is static, unless you are doing something with malleable terrain, in which case i don't know.