* sein: This is the verb "to be," and is used for the third-person plural ("they are"). For example, "Sie sind" (they are). It can also be used for the formal "you are" (Sie sind).
* ist: This is the verb "to be," and is used for the third-person singular ("he/she/it is"). For example, "Er ist" (he is), "Sie ist" (she is).
* sind: This is the conjugated form of "sein" used for the plural "you are" (informal: ihr seid; formal: Sie sind) and "they are".
To give you the correct German word, I need the complete English sentence you're trying to translate.