牙齿 (yáchǐ) is the general, slightly more formal word for 'tooth/teeth' as a set or concept. 牙 (yá) is the everyday spoken word for a single tooth or the idea of teeth, and is also used in compounds like 牙刷 (toothbrush) and 牙膏 (toothpaste). Use 牙齿 when talking about dental health or anatomy; use 牙 in casual talk about a specific tooth or in compound words.