When we conjugate a radical changing verb, both the stem and the ending change, as opposed to just the ending changing as it does for 'normal' verbs.

The stems change for the 'I', 'you', 'he/she/it' and 'they' forms, however the 'we' and 'you (plural)' forms conjugate as expected.  Due to this pattern they are sometimes known as '1, 2, 3, 6 verbs'.

There are three groups of radical changing or stem changing verbs : o -> ue, e -> ie and e -> i (for example cerrar meaning to close conjugates as cierro, cierras, cierra, cerramos, cerráis, cierran).