The Triffids themselves are huge, over twelve feet tall, and are armed with powerful stingers which deliver a deadly neurotoxin which kills almost anything after coming into contact with its flesh. Triffids are vulnerable to fire and electricity but do not need to feed or sleep, as they can photosynthesise. They are, however, capable of devouring flesh for extra nutrients.
They were farmed on Earth for a while, as their stingers secrete an nectar which is rare and valuable. However, when the Triffids started to multiply (nearly resulting in a mass outbreak) they were transported away from Earth. Thousands of transports were involved in the relocation, but they were fitted with inadequete facilities to contain the creatures and so many of the transports suffered outbreaks and were overrun. The Human fleet destroyed several of the transports but some were able to slip away from the solar system, never to be seen again.
The Triffids have been shown to possess limited intelligence as they hunt in packs and some even utilise tactics like ambushes and flanking maneouvers. They supposedly also gained the ability to somehow control the transports they had captured, and have now presumably colonised their own world.