Amateur radio is a hobby where people communicate over radio frequencies using own radios and antennas. Large power can be used and operation requires passing a license tests. Licensing is carried by the ARRL organization.
The communication can be from home operated radios with small or big antennas, from mountaintops with portable radios and wires, over amateur radio satellites, or via reflection from the moon.
Radio amateurs form a close knit community worldwide. When anywhere in the world I see a house with a ham radio tower, I knock in and I am welcomed like an old friend.