Wat is Cox–Ross–Rubinstein and Jarrow–Rudd trees?

The Cox–Ross–Rubinstein tree takes $u=e^{\sigma\sqrt{\Delta t}}$ and $d=1/u$, so that the tree is symmetric in log-price and the drift enters only through $p$. The Jarrow–Rudd tree takes $u,d=\exp\bigl((r-q-\tfrac12\sigma^2)\Delta t\pm\sigma\sqrt{\Delta t}\bigr)$, which puts the drift into the factors and makes $p$ close to one half.