The things that make these objects magnetic is that they all contain iron, nickel or cobalt.
A magnetic field is a area of land where inside of this piece of land you would be able to detect magnetic forces. The tighter the lines are in a magnetic field the stronger the magnetic force would be in that area.
You can make an electromagnet with a coiled piece of copper wire and an eletric current. You can make electromagnets stronger by adding an iron bar into the coil. The bar then becomes magnetised. You can also make the electromagnet stronger by either highering the voltage or putting more coils onto the wire.
The advantages of an electron magnet is that you can turn it on and off. So for example if you have paperclips attached to the magnet with an electromagnet you just switch it off and they all drop off but with a bar magnet you have to pick them off one by one.Magnets can be used for fridge doors, car doors, fridge magnets and electromagnets are used in rollercoasters, junk yards, eletric bells.
The domain theory is that inside a magnet the domains are all pointing the same way which causes it to repel other magnets which domains are pointing against it but will attract magnets where the domains are pointing the same way. If you drop a magnet it may cause the domains to be jumbled up and lose its magnetisation