These verbs are known as stem-changing or radical-changing verbs, because what makes them irregular is that only the stem (the part of the verb that comes before -ar, -er or -ir) changes.
For example, any form of the regular verb hablar, whatever the tense or mood, starts with habl-. But the verb volar, which usually means "to fly," can have its stem change. Fortunately, the stem of verbs like volar change in a predictable way, and that is that the o changes to ue whenever it is stressed. When them stem isn't stressed, it remains unchanged.
Thus, the singular present indicative forms all change: vuelo (I fly), vuelas (you fly), vuela (he/she/you fly). But the first- and second-person plural forms do not: volamos (we fly), voláis (you fly). The third-person plural form changes: vuelan (they fly).
The same pattern — the o changing to ue when stressed — follows for all other forms. You can see how this works by looking at the complete conjugation of contar.
Other verbs that are conjugated in exactly the same way are acordar, acostar, apostar, comprobar, consolar, costar, demostrar, encontrar, mostrar, probar, recordar, rodar, soltar, sonar and soñar.
A similar pattern can be seen with verbs such as mover. The stem of mover changes in the same way as do the stems of voler and contar, but the verb has some differences in conjugation because it is an -er verb rather than an -ar verb. Verbs that are conjugated in the same was as mover include conmover, doler, promover, remover, and soler. The same goes for dormir, which, except for the stem, follows the pattern of other -ir verbs.
There are a few verbs that have the o-to-ue stem change but are also irregular in other ways. For example, morir is conjugated in the same way as dormir, except morir also has an irregular past participle. And oler follows the pattern except than an h is added at the beginning of the word when it is stressed on the first syllable.
