For example, in English, the nominative and accusative forms of you are the same, whereas he/him, she/her, etc., have different forms depending on grammatical case. In Latin, the nominative and vocative of third-declension nouns have the same form (e.g. rēx "king" is both nominative and vocative singular). Similarly, in German, the infinitive, first person plural present, and third person plural present of almost all verbs are identical in form (e.g. nehmen "to take", wir nehmen "we take", sie nehmen "they take"). In the Finnic languages, such as Finnish and Estonian, there is syncretism between the accusative and genitive singular case forms, and the nominative and accusative plural case forms.
Syncretism can arise through either phonological or morphological change. In the case of phonological change, forms that were originally distinct come to be pronounced identically, so that their distinctness is lost. Thus in the German case, the infinitive nehmen comes from Old High German neman, the first person plural nehmen comes from nemēm, and the third person plural nehmen comes from nemant. In the case of morphological change, one form simply stops being used and is replaced by the other: this is the case with the Latin example, where the nominative simply displaced the vocative in the third declension.
Baerman, Matthew; Dunstan Brown; Greville G. Corbett (2005). The syntax-morphology interface: a study of syncretism. Cambridge: Cambridge University Press. ISBN 0521821819.