Usually a 3-year renewable contract. So often, who gets hired is a matter of luck. Typically, each of these has applied to 5-10 schools. A good professional lineage is essential. A note about medical schools vs. biology departments.
Salaries are lower in biology, but tenure is more secure.
In medical school departments, tenure is almost meaningless. Your salary is absolutely tied to your funding. Also medical school departments with MDs will trade off their MD-teaching responsibilities for clinical service. This can lead to a 2-tier system of faculty treatment. Two of my friends, teach at small colleges in Georgia.