The green leaves of the rhubarb contain large amount of oxalic acid and probably toxic anthraquinone glycosides. Eating the leaves can lead to acute poisoning and also death. The anthraquinone glycosides are thought to be the main cause for rhubarb leave poisoning.
On the website of the Canadian Poisonous Plants Information System you find the following about rhubarb:
> The stalks are widely used as preserves and are also eaten raw, without problems. The toxic content is much lower in the stalks. Humans have been poisoned after ingesting the leaves. Human poisoning was a particular problem in World War I, when the leaves were recommended as a food source in Britain.
To summarize, the rhubarb _stalks_ are safe, but you should never eat the rhubarb _leaves_.