Taxol is a compound originally obtained from the bark of the Pacific Yew tree
Taxus brevifolia, an "environmentally protected species". It was discovered following a screening programme, in which it was found to have the ability to slow down or prevent growth of cancerous tumours.
However it is now produced by complex chemical manipulation of a similar compound from a related species
Taxus baccata which is much more widely available.
This appears to be due to its action on microtubules which make up the spindle, which is involved in mitosis which precedes cell division. Presumably the molecular structure of taxol causes it to bind to microtubules. By preventing mitosis, it will affect rapidly dividing cells such as cancer cells more than other ordinary cells which are not dividing.
Under the trade name of paclitaxel it is being used to treat ovarian, breast and certain lung cancers.