Hot glue is a form of solid plastic that when heat is applied turns into an adhesive and then cures on cooling. You'll often see this kind of glue in hobby shops.
More specifically, hot glue is a hot melt adhesive, or a thermal plastic adhesive. The glue itself comes in the form of a solid plastic, somewhat flexible, but non-sticky cylinder that's often placed into the back of a glue gun. The glue gun is there to provide the heat to change the viscosity of the glue to allow it to be applied. When its temperature reduces back down to room temperature then it is set in place.
What jobs is it good for?
Hot glue is a lovely, forgiving adhesive which is safe to use in almost any environment, albeit it may not give you a very strong bond.
This makes it very popular in arts and crafts and in bonds that do not need a high pulling strength. Typical uses will be putting together softwood boxes that might be intended just to sit on a shelf and hold a few things and so the glue is there simply to hold it together. Book spines that need to be flexible but wouldn't have any strong pull on them are another typical application as is applying decorations onto wood.
Hot glue does have the benefit, so long as it's not a strong bond that's needed, of being able to glue almost anything. It does benefit from a porous surface, but equally it can bind a non-porous surface, albeit with lower strength.
What jobs is it not good for?
The fact that hot glue is a thermal plastic adhesive means that when it's viscosity increases because of heat, it's not going to hold the bond. For any environment where raised temperatures or raised pressures are involved then, it's not going to hold.
It doesn't do too well with solvents either. So in any environment where its going to come in contact with solvents you need to choose a different adhesive.
And finally, as I mentioned above, this is not a glue that you should have any faith in to hold structural tension or to hold a lot of pressure or weight. It's not intended for that purpose.
How to use
This is a very easy glue to use. Put it in the glue gun. The glue gun heats it up. The molten glue comes out the front. Hold the surfaces together until it cooles, typically for a couple minutes, and you have a bond. The curing time has more to do with the ratio of the temperature of the room versus the temperature of the glue.
However, this is a glue that I regularly allow my daughter to use and I've yet to regret that.