A specific, memorizable set of verbs — mostly describing motion or a change of state — takes être rather than avoir in the passé composé: aller, venir, arriver, partir, entrer, sortir, monter, descendre, naître, mourir, rester, retourner, tomber, and their compounds like revenir and devenir. This set is often taught with the mnemonic house "Dr & Mrs Vandertramp", one letter per verb.
Reflexive verbs also always take être, regardless of meaning: elle s'est levée (she got up), ils se sont parlé (they spoke to each other). With every être verb, the past participle agrees in gender and number with the subject, the same way an adjective would: il est allé, elle est allée, ils sont allés, elles sont allées.