The origin of the Triffid species was hinted at by Wyndham, but was never fully revealed. 20+ years prior to the events of Day of the Triffids, the original Triffid seeds were discovered in a box inside an aircraft that was shot down when its crew attempted to defect during the cold war.
Wyndham's narrator speculates that the Triffids may have been a bioengineered hybrid species; created using material from several other plants, by the real-life Soviet biologist Trofim Lysenko. This comparison that is fueled by Wyndham describing the Triffids by breaking them down into their components parts, and comparing these parts to existing plants. The narrator quickly dismisses the idea that Triffids are a naturally occurring species, or that they are aliens. In the 1962 film adaptation, the Triffids were a form of extraterrestrial plant life whose seeds were spread across the globe during a meteor shower.
The base of a Triffid is a large muscle-like root mass comprising many thick tentacles. When dormant/docile, these tentacles are rooted into the ground and are used to draw nutrients, as with a normal plant. When active, Triffid use these tentacles to propel themselves along at a moderate walking pace. They are capable of moving faster over open ground. Triffids Roots/tentacles are sufficiently articulate to allow them to climb stairs, and strong enough to allow them to push through fencing. They are not, however, prehensile enough to allow them to use tools.
Triffids are plant-based and their vital functions rely on distributed systems instead of distinct internal organs. This makes them difficult to kill using firearms and allows them to absorb considerable blunt force damage to their body section without being impaired. Triffids are also capable of limited bodily regeneration, and can regrow their sting if it is damaged or destroyed."
The most effective way to stop or disable a Triffid is to sever its trunk or otherwise destroy its head, or just simply overwater it.