A humidifier is basically a thing with a water container, and then usually a wheel like a "water wheel" on an old mill. The water wheel is covered with a sponge that soaks up the water as it rolls into it, and then a fan blows thru the sponge when it is out of the water, making the water evaporate and get blown into your house, so the air in your home is less dry. Some come with humidstats that you can set so it turns on and off by itself and keeps your house (or the area near it) at a fairly constant humidity level.
That said, I find they are noisy and they drive me nuts. I like a different method that works really well in our house. We have a fountain on an end table in the living room, and another one in our bedroom. They make cool noises, and yes, you HAVE to remember to fill them almost daily (probably daily when you first set it up) Just the fact that there's running water in the fountain and dry air overhead does the same thing as a humidifier. Fountains splash a bit, so you have to protect the surface nearby, but I think they're kind of neat, so we use them in the winter. (Although I stopped using the one in the Living Room, because we have a fish tank in there, and it does the same thing.
On the heat issue, a humidifier does add heat. It's got an electric motor and the motor does get warm. (The fountains have wee little electric pumps, so they probably get warm, too, but they are little, so we don't notice it)