A glowstick is a sealed, flexible plastic tube which contains a chemical compound. Nested inside the flexible tube is capsule of brittle plastic containing a different compound. When the glowstick is bent sufficiently, the capsule breaks and the two chemicals mix and produce the eponymous glow. Glowsticks were once exclusively green, but now come in a variety of colours.
Glowsticks produce no fumes, no heat, are almost unbreakable, and their glow lasts for many hours (usually all night). Should a glowstick be breached, however, the internal chemical fluids should never be consumed, as they can make you quite sick.
Glowsticks are also used for visual effect in certain mixed drinks, and sometimes at late evening archery events, where very thin glowsticks are taped to the shafts of arrows, providing an interesting and dramatic light show.