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?

While its probably more suited for /prog/- although they're nothing but trolls- you'd probably have things like hitboxes and collision detection for any sort of platformer, so typically you'd have a variable that stores y-velocity for a given entity, and 'gravity' would simply add a static amount to it every update. You could even have a pseudo air-resistance by making a maximum downward velocity and such, but this is all stuff you can learn from tutorials and not so much from a math and science board.