Yew is the garden shrub whose red, berry-like structures contain a poisonous seed inside a fleshy red cup. The red cup is technically an aril rather than a true berry, and unlike most of the plant, the aril itself is generally not poisonous; the seed inside is.
Yews contain taxine alkaloids, especially in their leaves and seeds. These toxins can disrupt calcium and sodium channels in the heart, making ingestion potentially fatal. The seeds are bitter, but chewing can release their toxins, so even ornamental yews should be kept away from children and animals.
Yew has been planted for centuries in parks, gardens, and churchyards, particularly in Britain and Ireland. Birds can eat the red arils and disperse the seeds, often leaving the hard seed intact. A common mistake is calling the red structure a berry: botanically, it is a modified seed covering.