Evaporative humidifiers warm up the water so that it evaporates, the same way as if you leave a tray of water out, but faster.

Cool mist humidifiers blast the water out into the air using ultrasonic vibrations. I find that this is a much more effective way of getting moisture into the air, but it has one big drawback. All the hardness in the water goes with it and eventually it settles out as a fine white dust all over everything.

For growing plants, you shouldn't need a humidifier. Plants produce their own humidity, directly from the leaves, from wet soil, or from watering trays. Your only job is not to lose it. Cover plants with humidity domes until germination and then remove them gradually to toughen up the plants. Misting seedlings once or twice a day is much better than constantly having high humidity.