Torch is the light source every world starts with, and the one that decides where mobs can appear. Hostile mobs need block light to be entirely absent to appear, so a single Torch shuts down spawning across a surprising radius — lighting a cave is less about brightness than about leaving no fully dark spot anywhere.
Running out of coal does not stop you. Burning any log in a furnace produces charcoal, which makes torches the same way coal does, and a stack of logs lights a long stretch of tunnel. A Torch breaks instantly by hand and drops itself, so a trail of them costs nothing to pick back up on the way out of a cave.
They sit on top of blocks and against the sides of most of them, but never underneath, and they do not survive water — a wall torch pops off the moment a flow reaches it, so lighting a flooded cave or an underwater build needs glowstone, sea lanterns or a jack o'lantern instead. A Torch placed beside ice or snow melts it, which is worth remembering before you light an igloo.
Three relatives change the job rather than the light. A soul torch, made with soul sand or soul soil, burns blue and keeps piglins away from a Nether outpost. A copper torch burns green and never oxidises — the copper is fuel rather than structure — so it otherwise behaves like an ordinary Torch. A redstone torch is a different thing entirely: a power source that happens to glow, not a way to light a room.