All Spanish verbs have a stem (la raíz) and an ending (-ar, -er, -ir): pensar, volver, pedir. There is a large group of verbs that change their stem in the present tense when the stem vowel is stressed. There are three types: those that change -e to -ie-, those that change -o- to -ue- and those that change -e- to -i-:

Common verbs following these patterns in the present tense:
e > ie: cerrar, comenzar, empezar, entender, pensar, perder, preferir, querer, sentir
e > i: elegir, (im)pedir, seguir, servir, (son)reír
o > ue: contar, dormir, encontrar, morir, mostrar, poder, resolver, volar, volver

Jugar has a different stem change, from u to ue in the same places:
juego, juegas, juega, jugamos, jugáis, juegan.

Note: All -ir stem-changing verbs change their stems in other tenses (see appendix A).