1.- Different types of light (infrared, microwaves, photons, X-rays, etc... ) all interact with molecules in different ways.

Imagine chemical bonds between atoms in a molecule as little springs. In a real molecule at room temperature, the atoms are always bouncing back and forth on those springs randomly. When an infrared photon hits a molecule, the photon is absorbed by the molecule, and all that energy goes causes the atoms to vibrate more, like in this gif: atoms vibrating

Those vibrations cause the substance to heat up.
Microwaves, on the other hand, heat up molecules by making them spin around.

2.- In liquid water, all of the atoms are vibrating around within individual water molecules, and the water molecules are moving around and spinning randomly. Each molecule also has a weak attraction to the other molecules around it. All of these weak attractions between all of the different water molecules in a cup of water keep the water together as a liquid. Since the movement of all the molecules is random, some molecules are moving faster than others. Some are even moving so fast that they can break free from their attractions to the other water molecules and fly away from the cup of water. These molecules have become part of the gas in the room. As you heat up the cup of water, the molecules of water in the cup all start moving, vibrating, and spinning faster and faster. More and more molecules have enough speed to escape the liquid water and become gas. Finally, at 100°C, all of the molecules in the cup are moving fast enough to escape the liquid, and the liquid all boils away into gas.