Être auxiliary

There are only two auxiliaries in French: avoir and être.
As avoir, être is used to form compound tenses for some verbs.
One says je suis tombé or je suis venu. It is the normal conjugation of this verb. Avoir is never used for those verbs.
Verbs with this particularity are called verbes d'état (status verb), ie there is no action but only a state. Here is a few example of those verbs:

mourir naître tomber devenir
venir partir arriver aller
...

This list is not meant to be exhaustive and there are many other status verbs.

There is, however, a mnemonic way to remember verbs that take the auxiliary being in the past tense. This is mostly taught for people learning French but it is interesting even for a French speaker. We must remember DR MRS VANDERTRAMP. Each of the first letters corresponds to a status verb:

		Devenir
		Revenir
		Monter
		Rester
		Sortir
		Venir
		Aller
		Naître

		Descendre
		Entrer
		Retourner
		Tomber
		Rentrer
		Arriver
		Mourir
		Partir

Être with passive forms

Être can also be employed with transitive verbs ie followed by a direct object complement (COD) in order to give to the subject a passive role. Active: le chat mange la souris (cat eats mouse)
Passive : La souris est mangée par le chat. (mouse is eaten by cat)

When auxiliary être is employed in conjugation, subject takes action meanwhile it is the actor of this action with auxiliary avoir. This change is not possible in case of verb is not followed by a direct object complement (COD). By changing the sentense to passive, COD becomes an agent complement.

One important remark, there is a link between the subject and past participle at passive form.